We describe a case of a giant unhomogenous uterine leiomyoma that simulated ovarian cancer both clinically, roentgenologically (computerized tomography) and ultrasonically. As an incidental finding there was also a carcinoma in situ of the endometrium. The difficulties in the differential diagnosis of an abdomino-pelvic mass are discussed, and the significance of laparotomy also as a diagnostic method is emphasized.