Multicystic renal dysplasia was found in a 6-day-old Japanese black bull. Grossly, both kidneys were markedly small (2.0 x 3.5 cm) with numerous cysts ranging from 1 to 8 mm in diameter. Histopathologically, both kidneys consisted of many irregularly enlarged cysts, immature glomeruli, small ducts and anomalous stromal connective tissues containing focal persistent mesenchyme characterized by a proliferation of stellate cells with myxomatous area. These features are compatible with those of multicystic renal dysplasia in humans and other mammals.