Quantitative data on familial cancer risks are important for clinical, psychological and scientific reasons. The available estimates carry many uncertainties due to sample size and possible bias in data collection and often refer to first-degree relatives of unspecified age and sex. We calculated sex- and age-specific familial hazard ratios (FHRs) of cancer in offspring aged 15–53 years of cancer probands at 16 male and 17 female cancer sites, based on registered nation-wide data, free from bias. The familial risks in offspring were high, >5 for thyroid (FHR 10.7 in all offspring, CI 95% 6.9–16.6), and testicular cancer (FHR 5.4, CI 95% 2.6–11.3), or intermediate, FHR 2–5, for colon, rectal, lung, breast, cervical, uterine, ovarian, skin (melanoma and squamous cell) and other endocrine gland cancers. FHRs <2.0 were observed for stomach, renal and nervous system cancers, lymphomas and leukemias. Some sex differences were observed: FHRs for male breast (only 2 cases) and thyroid cancers were over 2 times higher than the respective female ones. When parents were diagnosed before age 50 years, offspring were at an increased risk of familial breast, renal, skin (melanoma), nervous system, thyroid and non-thyroid endocrine gland cancers, particularly affecting young (<40 years) individuals. The parental diagnostic age also affected offspring's risk of colon, rectal, uterine and ovarian cancers, but young individuals were not at a particular risk. No effect of age was noted for cervical cancer and lymphoma.Int. J. Cancer 78:172–175, 1998.© 1998 Wiley-Liss, Inc.