The results of classical segregation analysis on 159 families with polycystic kidney disease (PKD) are presented. It had been previously estimated that about 95% of autosomal dominant PKD (ADPKD) families have PKD1, the gene localized to chromosome 16p. The main purpose of the study was to determine if PKD shows any segregation distortion and to obtain new estimates of the age-dependent penetrance. Penetrance at the early ages of onset has increased during the last decade, presumably because of improvements in renal imaging and consequent earlier age of diagnosis. In the current study, the mean age of diagnosis was estimated to be 20 years, with a standard deviation (SD) of 15.94. Under the best fitting model (autosomal dominant), over 70% penetrance was estimated by age 30 years, over 95% by 50 years, and 99% by 55 years. Thus, diagnosis of this disease at an early age is possible without total reliance on DNA typing. The segregation ratio defined through the transmission probability in our model was not significantly different from 0.50, but its confidence limits were broad: 0.36 to 0.64. Neither transmission probability nor penetrance was significantly influenced by gender. The mutation rate was estimated to be 6.9 x 10(-5), in accordance with the previously observed high mutation rate for PKD. However, the mutation rate in our study may be overestimated because it neglects low penetrance alleles and phenocopies.