Contralateral ovaries from patients with unilateral ovarian carcinoma were examined and compared to ovaries from age-matched control patients without ovarian carcinoma. The number of inclusion cysts were increased in ovaries from patients with ovarian carcinoma compared to the controls (p < 0.01). In addition, inclusions from cases with ovarian carcinoma showed serous differentiation more frequently than the controls (p < 0.01; odds ratio = 10.0; 95% confidence interval = 1.2-78.1). An age-related increase in the number of inclusion cysts was seen in the study group but not in the control group. These findings support a role of surface inclusion cysts in the genesis of ovarian carcinoma.