Primary mucinous cystadenomas are rare retroperitoneal tumors that are exclusively found in women. The histogenesis of these tumors is still uncertain, though mucinous metaplasia of the mesothelial lining cells is the most likely hypothesis. We report a 19-year-old pregnant female, initially believed to have a severe hydronephrosis of left kidney, but was finally diagnosed as a retroperi- toneal mucinous cystadenoma concomitant with an atrophic left kidney.