We used the nationwide Swedish Family-Cancer Database to analyse cancer risks in women who have had children with different men. Cancer cases were retrieved from the Swedish Cancer Registry for 1961-1998. A total of 3 million women and 316 497 cancer cases were covered. For women having children with more than one partner, an increased risk was shown for upper aerodigestive tract, anal, liver, pancreatic, lung, cervical, other female genital, kidney and urinary bladder cancers. A decreased risk was observed for breast and endometrial cancer, and melanoma. In women who had at least three or more children with three or more partners, the increased risks were even more pronounced for pancreatic, lung and cervical cancer. Conversely, the risk for breast and endometrial cancer, and melanoma was decreased. The present results indicated that women who had children with multiple partners showed an excess of smoking-related and sexually transmitted cancers. The decreased risks for breast and endometrial cancer and for melanoma were possibly related to lifestyle factors connected with economic deprivation. The magnitude of the effects was so large that failure to consider the number of partners may introduce bias.