There is a familial aggregation of prostate cancer, and 5 to 10% of all prostate cancers are estimated to be inherited in an autosomal-dominant mode. A population-based cohort study was performed in order to study familial prostate cancer and associated malignancies. A nation-wide register cohort study was conducted using an unselected study population. The cohort of 5,595 sons and 5,089 daughters of Swedish men found to have prostate cancer between 1959 and 1963 was identified. All types of cancer reported between 1958 and 1992 in this cohort were identified through linkage to the Swedish Cancer Registry. The expected number of different cancers was calculated using incidence rates obtained from the Registry. A highly significant increased overall standardized incidence ratio (SIR) of 1.65 (95% CI, 1.49-1.83) was obtained for prostate cancer, with 370 observed cases compared with 224 expected prostate cancers. The SIR was 3.18 among cases 45 to 49 years old at diagnosis, with the risk gradually decreasing to a SIR of 1.45 among cases over 80 years of age. Among sons and daughters with a father whose prostate cancer was diagnosed at an early age (