Unilateral renal cystic disease (URCD) is morphologically indistinguishable from autosomal dominant polycystic kidney disease (ADPKD) except for its unilaterality. Unlike ADPKD, URCD patients show neither a genetic background nor progressive deterioration in renal function; thus, the differential diagnosis of URCD from ADPKD is important. Only a few cases of URCD have been reported. This study reports two cases of URCD in adults together with a literature review. We identified these two cases using abdominal computerized tomography and family screening with renal ultrasonography.