What if 3 to 6 million people in the United States have an asymptomatic disease that is easily detected by CT and MRI but is often discovered incidentally? What if there are a variety of highly effective but expensive treatment options for this disease? Depending on the size, location, and duration of the lesion, these available treatments can prevent catastrophic events in 0 to 53% of cases. It is especially important to determine which patients should be treated, which patients should be closely monitored, and which patients should not be worried. This disease is the asymptomatic saccular unruptured intracranial aneurysm (UIA), and the management of this condition in clinical practice truly presents a dilemma for every neurologist. At the 2011 meeting of the American Academy of Neurology, Dr. Robert D. Brown, chief of neurology at Mayo Hospital, noted, “This is a fairly common condition. About 2 percent of Americans have UIA, and probably 14 of the 700 attendees here have the disease.” But he also notes, “Don’t worry, it doesn’t mean that all 14 of these people have a ticking time bomb in their bodies.” The annual incidence of aneurysmal subarachnoid hemorrhage in the United States is about 6 to 10 per 100,000, with a 30 to 40 percent mortality rate, Dr. Brown notes, “But most aneurysms don’t rupture.” Most natural history of disease studies agree that aneurysm size is the key factor in assessing the risk of rupture, followed by the location of the aneurysm and the patient’s age, with posterior circulation aneurysms and aneurysms in older patients likely to be at greater risk of rupture. In addition, risk factors for aneurysm rupture may include smoking, hypertension, alcohol consumption (higher risk in non-drinkers or heavy drinkers), family history, morphologic features of the aneurysm and its growth. However, as CT and MRI scans become more likely to detect very small aneurysms, the question is no longer “What can we do for young patients with incidentally detected aneurysms?” but rather, “Should we do this?” In the absence of well-designed randomized trials, Dr. Brown said, epidemiologic cohort studies can help guide the clinical management of aneurysms. For example, in the ISUIA (Worldwide Study of Unruptured Intracranial Aneurysms) trial, which included 5,500 patients, data from non-surgical patients showed that small aneurysms located in the anterior circulation of the Willis ring and the cavernous sinus segment of the internal carotid artery were very unlikely to bleed during conservative follow-up of 5 years. Data from this trial also suggest that even small (<7 mm diameter) aneurysms may rupture if located in the posterior communicating artery or posterior circulation, and are not uncommon. Ongoing and planned studies will explore whether more precise analysis of such aneurysms can more accurately predict the risk of rupture, such as the curvature, elliptical index, and nonspherical characteristics of the aneurysm. Computational fluid dynamics based on engineering principles may also have some guidance. Dr. Brown also noted that there is "considerable variation" in the management of small unruptured aneurysms in clinical practice, with some hospitals treating 90% of cases with spring coils or aneurysm clips and others treating less than 10% of such cases. Based on the relevant literature, Dr. Brown's own experience is to recommend treatment in young patients with aneurysms ≥7 mm in diameter who are in good health. 2. Young, healthy patients with aneurysms <7 mm in diameter but located in the posterior circulation. 3.Older patients with aneurysms between 7 and 12 mm in diameter and located in the posterior circulation can be treated. 4. Elderly patients with aneurysms >12 mm in diameter and with reasonable treatment options available. Dr. Brown recommended that all patients with unruptured aneurysms should be treated aggressively for hypertension and quit smoking, and that patients who are managed conservatively must also be followed closely based on imaging, given the limited data suggesting that moderate and large lesions grow over time. He noted that in a trial that included 165 patients (Stroke 2009;40:406-11), 1 of 12 small aneurysms (<8 mm in diameter) showed "significant growth" over 4 years. Based on forthcoming data from the ISUIA trial, there was a "strong and significant" downward trend in the risk of aneurysm rupture in those taking the highest dose of aspirin, so aspirin therapy may be beneficial for this group of patients.