Bilateral ovarian neoplasms are believed more likely to be malignant than are unilateral neoplasms. To our knowledge, this clinical tenet has not been evaluated or quantified. To test this hypothesis, we conducted a 10-year review of ovarian neoplasms found at the time of celiotomy at the Women's Hospital, Los Angeles County-University of Southern California Medical Center. Overall, women with bilateral neoplasms had a 2.6-fold increased risk of malignancy when compared with women who had unilateral neoplasms (95% confidence interval, 2.0 to 3.4; p less than 0.001). The age-standardized relative risk was 1.8 (95% confidence interval 1.4 to 2.3). When standardized for the effect of menopausal status, the relative risk was 2.0 (95% confidence interval, 1.6 to 2.5). Bilaterality is associated with a doubling in the risk of cancer, a highly significant difference.