Consideration of cerebral arteriovenous malformation (AVM) illustrates how decision analysis may be used to evaluate systematically two treatment options--surgery before bleeding occurs or conservative management. The analysis balances risk of immediate surgical mortality and morbidity against the delayed risks inherent in conservative management. Because the condition is rare and local experience therefore scant, probability estimates have to be obtained from the literature. From these probability estimates and from estimates of attractiveness of various outcomes (made in this study by a clinician) a quantitative assessment of the risks and benefits of surgical versus medical management can be calculated. The finding is that conservative treatment is preferable to surgery in patients who present with an AVM before bleeding occurs, even when the probability estimates used are those obtained under conditions more favourable to surgery. The decision also remains unchanged when different attractiveness values are assigned to possible outcomes.