We evaluated risk factors for functional ovarian cysts using data from a case-control study conducted in Milan. Cases were 68 women with histologically confirmed functional cysts. Controls were 272 women admitted to hospital for acute conditions. Compared with women who were 11 years of age or younger at menarche, women who experienced menarche at 12-13 years or at 14 years or later had odds ratios (ORs) of 0.9 [95% confidence interval (CI) = 0.5-1.6] and 0.4 (95% CI = 0.2-0.9). In comparison with women reporting menstrual cycles lasting less than 26 days, the OR was 1.8 (95% CI = 0.7-5.0) in women reporting cycles lasting 26-30 days, 1.8 (95% CI = 0.4-7.4) in those reporting cycles greater than 30 days, and 1.9 (95% CI = 0.4-7.7) in those with totally irregular cycles. Body mass was inversely related to the risk of functional ovarian cysts: compared with women with a Quetelet index below 20, the OR was 0.9 (95% CI = 0.5-1.7) in those with an index in the range from 20-24 and 0.5 (95% CI = 0.2-1.2) in those with an index of 25 or more. We found little relation between the risk of functional ovarian cysts and education, smoking, marital status, or age at first marriage. For oral contraceptive use, we found an OR of 1.3 (95% CI = 0.7-2.6).