A rare case of placentae site trophoblastic tumor (PSTT) studied by immunohistochemistry and nuclear DNA analysis is reported. The patient, a 24-year-old Japanese female, complained of amenorrhea. Dilatation and curettage revealed a small specimen that contained trophoblastic cells and caused intractable bleeding. Pelvic sonography revealed a 5-cm mass in the posterior uterine wall with multiple cystic lesions of several sizes. The cystic lesions were shown to be dilated vessels by magnetic resonance imaging (MRI) and digital subtraction angiography (DSA). Serum beta-hCG (beta subunit of human chorionic gonadotropin) was 3.7 ng/ml. Total abdominal hysterectomy revealed a well-circumscribed, yellow, soft mass in the posterior uterine wall. Microscopic findings were consistent with PSTT and the mitotic count was extremely low. Immunohistochemically, most of the tumor cells were intensely stained with human placental lactogen, whereas few were stained with human chorionic gonadotropin. The nuclear DNA content of the trophoblastic cells showed a sharp peak at the triploid range coexistent with a few cells of higher ploidy. This is the first report of sonographic findings and nuclear DNA analysis by spot cytometry in a case of PSTT.