We examined clinical and sonographic findings in 112 patients diagnosed as having hemorrhagic ovarian cyst (HOC) who had clinical and transvaginal sonographic follow up. The patients were classified into group A (n=40) with signs and symptoms of acute abdomen and group B (n=72) with no symptoms or mild abdominal pain, and their ultrasonographic and clinical findings were compared. Significant differences were found in mean age, white blood cell (WBC) count, greatest diameter of the mass, shortest diameter of the mass, and size of cross section of the mass. The internal echograms of HOCs were grouped into 4 types: (1) hyperechoic and hypoechoic solid type; (2) reticular or sponge-like type; (3) mixture type of solid and cystic components; and (4) cystic types. In all image types, septum-like or thread-like echoes were seen. Transvaginal sonography (TVS) of type 1, type 2, and type 3 images showed a clear division into hyperechoic and other areas with the passing of time which was finally changed into a cystic pattern and disappeared. HOCs were found more frequently in nulliparous patients (n=79, 70.5%) than in multiparous (n=33, 29.5%). There were many luteal phase (n=86, 76.8%) in comparison with follicular phase (n=13, 11.6%). Thirteen cases were detected during early gestation (n=13, 11.6%). In group A, severe pain reduced or disappeared within 3 h in 37/40 (92.5%) of the patients. Blood flow inside the masses was analyzed in 14 patients by the color Doppler method and showed no significant change. Taken together, this study elucidated the ultrasonographic and clinical characteristics of HOCs, which provide useful information to differentiate HOCs from organic masses and help to avoid unnecessary laparotomy.