The clinicopathologic features of 2 cases of trophoblastic pseudotumor of the uterus are presented and compared with previously reported cases. In both cases, cancer was erroneously diagnosed. One patient, 27 yr of age when first examined, presented metrorrhagia 10 mon. after a full-term delivery. The lesion was entirely removed by curettage and was no longer found in the hysterectomy specimen. The patient is alive and well < 17 yr after therapy. The other patient, 36 yr of age at the initial examination, presented with metrorrhagia 23 mo. after a therapeutic abortion. The pseudotumor infiltrated the full thickness of the uterine wall. The patient is alive and well 3 yr and 7 mo. after hysterectomy and radiation therapy. Histologically, the tumor consisted of large cells invading the myometrium in small clusters as cords or as single cells. There were 2 mitoses/10 HPF of which some were atypical. Tumor cells containing chorionic gonadotropin (hCG) and a pregnancy-specific protein (SP1) were demonstrated immunohistochemically in both cases and a pregnancy-associated glycoprotein (.alpha.2-PAG) and placental lactogen (hPL) in 1 case. Ultrastructurally, trophoblastic pseudotumor closely resembled cytotrophoblastic cells of the basal plate of the placenta.