Five-month-old a female mongrel puppy weighing 3.5 kg showed no systemic disorder and particular discomfort except abdominal distension at the first visit. On physical examination an irregular abdominal mass was palpated. One month later she was clumsy and uncoordinated. In addition, lethargy and anorexia were appeared. Then she became comatose and died in spite of initial therapy. In radiographic examination enlargement of both sides of kidney was observed. The hematological examination the dog had WBC of 16,250/$mu$l, RBC of $7.2{ imes}10^6$ $mu$l, PCV of 32%, total protein of 8.0 g/dl, and fibrinogen of 900 mg/dl. In serum chemistry BUN was 87.4 mg/dl and creatinine was 5.1 mg/dl. Urinalysis revealed pH of 5.6, SG of 1.009 and protein of 500 mg/dl. In urine sediment test many RBCs, leukocytes, inflammatory cells and a few epithelial cells were observed. On histopathologic examination the size of right and left kidney were 15 cm, 16 cm in length, 6 cm, 6 cm in widths, respectively. Both sides of kidney were filled with brown-orange fluid and had irregular capsular surface. The cysts of various sizes were located throughout the cortex and medulla. No abnormality was found in any other organs. Histologically, cyst was lined by cuboidal to slightly flattened tubular epithelium and surrounded by mature fibrous connective tissue. Glomeruli, tubule and renal pelvis remained normal between cysts and exfoliated epithelial cells.