Histological findings in 75 children with polycystic kidney disease were analysed. The patients had been divided into recessive (RPKD), dominant (DPKD), and sporadic cases on the basis of family investigations. The histological findings in the patients with RPKD and DPKD were compared with each other in order to define criteria for classification of the sporadic patients. The kidneys of the neonatally-deceased patients with RPKD (26 patients) and DPKD (4 patients) were macroscopically very similar. On histological investigation, all patients with RPKD (32 patients) showed cysts derived from the collecting ducts. In DPKD, the cyst epithelium was more variable, and the presence of glomerular cysts served as a useful histological marker of this disorder. In RPKD there was diffuse biliary dysgenesis, in contrast to the normal liver in the neonatally-deceased DPKD patients. However, one DPKD patient who survived also had biliary dysgenesis. Using the observed differences between RPKD and DPKD as criteria, all sporadic patients with satisfactory histological material could be classified as RPKD. We conclude that the differential diagnosis between RPKD and DPKD can be based on kidney and liver histology.