Hemorrhagic ovarian cyst (HOC) which is one of the functional cysts, is often involved in acute abdomen leading to laparatomy intervention. The reason for this mainly lies in the fact that it is easily misdiagnosed as an organic mass because of the presence of lower abdominal pain and the variable appearance of ultrasonographic images at presentation. We analyzed 15 cases of HOC associated with acute abdomen, of which in 2 cases the disease was confirmed by laparotomy. The remaining 13 cases were followed-up clinically and by daily transvaginal sonography (TVS) from the first detection of the cyst until complete resolution. The TVS images showed a variety of changes; however, when the images or their magnified views were observed precisely, important diagnostic characteristics were found which were classified into 3 categories: type 1 images showed mixed hypoechoic and hyperechoic areas, the demarcation line between which appeared as a thin or thick septum-like echo of smooth formation; type 2 images showed hypoechoic background and vertical, horizontal, or lamellar thin or thick thread-like echoes with an overall reticular-like or sponge-like pattern; and type 3 images showed an overall hyperechoic and solid pattern. Type 1 and 2 images occurred more frequent (93.3%), and only 1 case had a type 3 image. In all image types, septum-like or thread-like echoes were seen, TVS type 1 and 2 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. Severe lower abdominal pain was present for 1 to 3 hours in 12 cases (80%), 4 to 6 hours in 2 cases (13.3%), and 11 hours in 1 case (6.7%). Other characteristics of HOC may be its most frequent occurrence in the young age group (10 to 20 years old, 80.0%) and in the luteal phase (84.6%). With operative cases, histopathological diagnosis was HOC. The clinical and particularly TVS findings described in the present study are of significant value in differential diagnosis of HOC with acute abdomen from other disorders presenting with acute abdomen.