The relationship between use of combined oral contraceptives and risk of epithelial ovarian cancer was investigated in a hospital-based case-control study conducted from 1979-1986 in seven countries. Three hundred and sixty-eight women with histologically confirmed epithelial ovarian cancer were compared to 2397 hospital controls matched for age, study centre, hospital and year of interview. The relative risk (RR) in women who ever used combined oral contraceptives, controlling for the confounding effect of number of pregnancies, was 0.75 (95% confidence interval (Cl) 0.56, 1.01). The relative risk decreased with increasing duration of oral contraceptive use and with increasing time since cessation of use. Except for mucinous tumours, oral contraceptive use was associated with lower risk of all histological subtypes of epithelial ovarian cancer. While the reduced risk associated with oral contraceptive use was present in parous women (RR = 0.85), it was most pronounced in nulliparous women (RR = 0.16). The magnitude of the negative association between oral contraceptives and ovarian cancer in five developing countries (RR = 0.77) was similar to that reported in developed countries.