Associations between reproductive factors and risk of ovarian cancer were examined in a prospective study of 60,565 women in Norway. A total of 445 women were diagnosed as having ovarian cancer during follow-up, from 1961 through 1980. The highest risk was observed among nulliparous women, and the risk decreased significantly with increasing parity. The estimated odds ratio for women with 5 or more births compared with one birth was 0.46, after adjustment for age, urban/rural place of residence and occupational class. Neither age at first or last birth, nor age at menarche or menopause, nor marital status, showed significant associations with ovarian cancer risk after adjustment for parity. The apparent protective effect of high parity was observed for epithelial as well as non-epithelial cancers, and for the separate histological types of epithelial cancer except mucinous cystadenocarcinomas.