This report concerns 347 patients with primary hydatidiform moles studied during the first 6 years (1966–1972) of operation of the Southeastern Regional Trophoblastic Disease Center. Aside from a decreased incidence, molar pregnancy in the United States follows a pattern similar to that elsewhere in the world. Abnormal bleeding is the key to early diagnosis, and the frequent use of sensitive HCG assays is the key to proper followup. Twenty percent of patients with hydatidiform moles can be expected to develop subsequent malignant sequelae. Bilateral ovarian enlargement and/or a large-for-dates uterus should alert the physician to a greter poential for this outcome. Spontaneous elimination of HCG from the circulation following molar pregnancy, as indicated by sensitive assay, would predict a benign postmolar course; no patient in the current series who once achieed undetectable levels of HCG developed malignant trophoblastic disease.