Pulmonary hypertension developed in two women who had been taking fenfluramine for over eight months for weight reduction. On withdrawing the drug symptoms and electrocardiographic evidence of pulmonary hypertension disappeared in both cases. In one patient, however, the evidence recurred after rechallenge with fenfluramine.These findings are strong evidence that fenfluramine may cause pulmonary hypertension. Hence any patient taking the drug should report immediately any deterioration in exercise tolerance.