We used the nation-wide Swedish Family-Cancer Database to analyse cancer risks in men who had had children with more than one woman. Cancer cases were retrieved from the Swedish Cancer Registry from years 1961–1998. A total of 2.9 million men and 298 134 cancer cases were covered. For men having children with two, three or more women, increasing risk trends were shown for upper aerodigestive tract, lung, urinary bladder and oesophageal cancers. Decreasing trends were observed for tumours of the colon, skin (squamous cell and melanoma), nervous system and endocrine glands and against myeloma and non-Hodgkin's lymphoma. The present results indicated that men who had had children with multiple women showed an excess of smoking- and alcohol consumption-related cancers. The decreased risks for colon cancer, non-Hodgkin's lymphoma and melanoma were possibly related to lifestyle factors connected with economic deprivation, less obesity and physical fitness. These ill-defined protected factors may be a challenge to epidemiological studies.