Several epidemiologic studies have shown familial aggregation of prostate cancer. To assess the nature of familial clustering of prostate cancer, a complex segregation analysis was performed on a population-based sample of 2,857 nuclear families ascertained through an affected father diagnosed with prostate cancer in Sweden during 1959-1963. The segregation analysis, using a large, unselected population of prostate cancer families, revealed that the observed clustering of prostate cancer was best explained by a high risk allele inherited in a dominant mode, with a high population frequency (1.67%) and a moderate lifetime penetrance (63%). This study confirms the result from one earlier published segregation analysis and provides the context for interpreting the recently published linkage of hereditary prostate cancer families to chromosome 1q 24-25 (HPC1).