Hydatidiform moles may be classified as partial or complete based on genetic and pathologic criteria. Between January 1979 and January 1990, 17 (5.5%) of 310 patients followed for partial mole developed persistent gestational trophoblastic tumor. Tissues from 14 partial moles were available for flow cytometric analysis of DNA content. Eleven partial moles (85%) were triploid, two (15%) were diploid, and one DNA histogram was uninterpretable. All patients with triploid partial moles achieved complete remission with one course of single-agent chemotherapy. The two with diploid partial mole required multiple courses of chemotherapy to achieve gonadotropin remission. Although the DNA content of most partial moles with persistent gestational trophoblastic tumor was triploid, diploid partial moles with persistent tumor were less sensitive to single-agent chemotherapy.