Anorectic agents, such as aminorex and fenfluramine derivatives, have been associated previously with the development of primary pulmonary hypertension. The combination diet drug phentermine-fenfluramine (or "phen-fen") was recently marketed in the United States. We describe a case of a 39-year-old woman with rapidly progressing, fatal pulmonary hypertension following administration of phentermine-fenfluramine. Autopsy was remarkable for complex pulmonary vascular lesions most consistent with thrombotic arteriopathy.