The death of John Rosnow last summer illustrates one of modern medicine's most preventable failings. A retired plant manager in Clintonville, Wis., Mr. Rosnow collapsed after a daily exercise regime that included riding his bike seven miles. Until then, "he was a fitness fanatic," says his widow, Joan Rosnow.

Yet the death of Mr. Rosnow, 70, was hardly inevitable. The aortic aneurysm that killed him had been growing for years, not days. Had he spent $40 to $150 on a noninvasive ultrasound test, his arterial bulge almost certainly would have been found. Then he could have undergone an insured surgical procedure that is 90% successful at repairing aneurysms.

Aneurysms -- bulges in weakened artery walls that are almost always fatal when ruptured -- may be the most preventable common killer that doctors rarely warn about. They account for more deaths in the U.S. than brain cancer or AIDS. And by all accounts, aneurysm deaths are underreported because sudden, unexplained deaths are often simply labeled "cardiac events." Yet despite the growing accuracy of screening technology, the medical community has never made much effort to detect aneurysms, and most general practitioners never advise patients to get tested.

That may be slowly starting to change. A federal health panel is considering whether to recommend routine screening for abdominal aortic aneurysms, one common form. And health insurers -- which have rarely paid for aneurysm screenings -- are starting to do so for patients with a family history of the condition.

The key to surviving an aneurysm is knowing about it before it ruptures. About 90% of ruptured aneurysms result in death. But about 90% of those fatalities could be prevented if the arterial bulge were detected in advance.

The advent of CT, MRI and ultrasound technology has produced an estimated 200% increase in the diagnosis of aortic aneurysms -- the most common variety. But nearly all these discoveries have occurred in patients who were being tested for something else. A radiologist scrutinizing film for gall stones can't help noticing if an aorta, typically the diameter of a garden hose, measures as large as a soda can.

So while radio advertisements boast that preventive CT and MRI scans can identify early-stage cancer and heart disease, another argument for getting a preventive scan may be to detect aneurysms. Though scans often fail to find cancer or heart disease -- or register false positives for those conditions -- they are brilliant at finding asymptomatic aneurysms. "There's no ambiguity with a CT scan in terms of aneurysms," says Michael Dake, chief of cardiovascular and interventional radiology at Stanford University Hospital.

Moreover, the value of a scan for aneurysms is longer lasting than that for other diseases, such as cancer. Aneurysms generally take years to develop, compared with certain cancers that can grow almost overnight. Adults at risk of aneurysms could undergo scans as infrequently as every five years.

Who Is at Risk

Research into aneurysm deaths has identified several groups at risk. All men over 50 who smoke -- or who once smoked for years -- should consider getting screened for abdominal aortic aneurysms, aneurysm experts say. All men over 65, even healthy nonsmokers, should get such a test, according to a recent study published in the British medical journal Lancet. People under 50 who should be screened include heavy smokers, those with family histories and those with recurring sharp pain in the head, stomach or back.

Perhaps the biggest high-risk group consists of the relatives of aneurysm victims. Research showing that people with aneurysms tend to share a genetic weakness in the walls of their arteries is relatively new but profound. At least 20% of aneurysm victims have a first-degree relative -- a parent, sibling or child -- with an aneurysm. Sufferers of a familial condition called Marfan's are especially at early risk of the condition.

But widespread awareness of this research has failed to penetrate the medical community, in part because few specialists focus specifically on veins and arteries. After a scan taken to investigate a heart murmur found an aortic aneurysm near his heart, Paul McGinley visited a geneticist and cardiologist, among other doctors. A sales representative for a large pharmaceutical company, Mr. McGinley had extraordinary access to medical experts. But nobody told him that his siblings -- including a sister who also has a heart murmer -- should be screened.

"It's absolutely crucial that close relatives get screened," says Carl Vaughan, assistant professor of medicine at Weill Cornell Medical College in New York, who has studied the family connection.

Most aneurysm victims are middle aged or older, creating a false impression among doctors that the condition never strikes the young. When the 19-year-old son of a middle-age aortic-aneurysm victim began experiencing chest pains, he was twice turned away from the emergency room on the mistaken belief that he was too young to have an aneurysm. He died, and other young members of his extended family have been found to have thoracic aortic aneurysms, says Dianna Milewicz, director of medical genetics at the University of Texas Health Sciences Center.

What to Request

Anyone obtaining a preventive scan should tell the radiologist or technician studying it to look out for aortic aneurysms, because attention tends to focus on organs rather than the artery supplying blood to all of them.

For smaller and harder-to-see cerebral aneurysms, it is best to skip the CT scan and get an MRI. Cerebral aneurysms are less predictable than aortic, whose chance of rupture grows with size. But the value of cerebral scans for identifying the need for surgery is great and growing, says Hunt Batjer, chairman of neurosurgery at Chicago's Northwestern Memorial Hospital.

Last month, Kevin Napolitano, a 40-year-old Maine father and construction manager, fell over dead from an unknown cerebral aneurysm. Dr. Batjer says the chances are "very good" Mr. Napolitano would be alive and well if he had had a scan.

Patients found to harbor aneurysms should demand to see a specialist, although this isn't as simple as it sounds. Which specialist to visit depends on the location of the bulge. Cardiologists and cardiac surgeons follow aortic aneurysms in the chest, while vascular surgeons treat those that appear elsewhere -- except in the head, where neurologists and neurosurgeons take over. Within these specialties, it is important to find individual doctors with deep aneurysm experience; many cardiologists, for example, know little about the condition.

Sometimes aneurysms rupture before reaching the recommended size for surgical repair. But even in such cases, victims who have had scans and are aware of their condition face better odds. That is because emergency-room doctors misdiagnose rupturing aneurysms about 30% of the time, studies show, often resulting in death. By contrast, the patient who enters the emergency room knowing he or she has an aneurysm can go straight to surgery.

One reason doctors have paid so little attention to aneurysms is that virtually nobody is sick with them. They are asymptomatic, and then they kill.