Only two examples of the entity of chorangiocarcinoma, in which there is a proliferation of both the vascular and epithelial components of the placental villi, have been reported in the literature. To test the hypothesis that chorangiocarcinomas are actually more common than implied by the literature, histological sections of chorangiomas were reviewed. Syncytiotrophoblast and cytotrophoblast proliferation, with nuclear atypia, similar to that found in trophoblastic neoplasia, were seen in 15 of 23 cases. Thus, 65% of chorangiomas fulfilled the diagnostic criteria to warrant re-assignment as "chorangiocarcinoma". The proliferation index of the cytotrophoblast in the tumor as measured by MIB-1 (Ki-67) immunostaining was significantly higher in "chorangiocarcinoma" than chorangioma (35.4% vs 15.7%, P<0.02). The following factors had no relationship to the presence or absence of trophoblastic proliferation: vascularity, cellularity, infarction, size or location of the chorangioma, or age of the patient. Five of the 15 chorangiomas with trophoblastic proliferation were of the chorangiomatosis variety. No formal follow-up was performed, as this was a retrospective study, but there is no recorded case of persistent gestational trophoblastic disease in this cohort, although one woman with "chorangiocarcinoma" had a history of previous hydatidiform molar pregnancies. An apparently benign clinical course is seen. These lesions, best described as chorangiomas with trophoblastic proliferation, are more common than suggested by the rarity of reported cases.