The clinical and pathologic characteristics of the murine polycystic kidney were examined in a mutant cy/cy mouse found in the KK strain, which is known to be a diabetic strain. This cy/cy mutant appeared to resemble human adult polycystic kidney disease in clinical course and morphology although inheritance was an autosomal recessive trait. Histologic studies of early postnatal animals suggested that cystic alterations might occur in any part of a collecting tubule or nephron. The cy/cy mutant might be a useful model animal for further investigation of early morphologic alterations and progression of cystic disease.