Identification of small ovarian benign cystic teratomas (dermoid cysts) is difficult and sometimes impossible by clinical findings alone. They may be missed even on direct inspection during laparotomy or laparoscopy. There are several typical ovarian sonographic signs which are known to occur frequently in women with benign ovarian cystic teratomas. High-resolution sonography can identify the cysts based on their typical sonographic signs. Since these very small dermoid cysts are sometimes undetected by the naked eye during the operation, ultrasonography with particular attention to the signs is very important. We present three cases reports that illustrate the problem and stress the contribution of preoperative sonography in the diagnosis and treatment of such cases.