Placental site trophoblastic tumor (PSTT), which can be regarded as a subtype of gestational trophoblastic neoplasia is discussed. The histopathological features include trophoblastic proliferation without the typical organization of the bilamelar cyto and syncytiotrophoblastic villus. Ultrastructural investigation has demonstrated a clone structural relationship between the infiltrating cells and those of the trophoblastic components of the normal human placenta. Clinical management should be based on the complete surgical resection of the mass and a follow-up by measuring the serum levels of beta-human chorionic gonadotropin fractions.