Although the significance of histologic grading in hydatidiform mole has previously been investigated, most studies evaluated patients treated before 1975. Since 1975, many advances have been made in the understanding and treatment of hydatidiform mole, including the division of molar pregnancy into complete and partial hydatidiform mole. We retrospectively studied 153 cases of complete hydatidiform mole diagnosed and treated at the Brigham and Women's Hospital between 1980-1990 to determine the current prognostic significance of histologic grading in this disease. The histologic grade (based on the criteria of Hertig and Sheldon) was compared with the subsequent clinical course, including the rates of spontaneous remission, persistent gestational trophoblastic tumor, metastatic disease, "high-risk" metastatic disease, chemotherapy resistance, and survival. The histologic grade of the original complete hydatidiform mole did not correlate significantly with any index of clinical outcome evaluated.