Congenital unilateral multicystic kidney is relatively rare in adult (16% in adult, and 84% in children). Most of the adult cases are asymptomatic, and should be followed up conservatively, but nephrectomy was performed in many cases reported in Japan, because it was difficult to distinguish those cases from renal tuberculosis. Here we present one adult case, and discuss its clinical findings, diagnosis, treatment, etiology and embryology.