In a double-blind 16-week crossover study, the effectiveness of verapamil therapy for chronic stable angina was evaluated in 19 patients (Phase I). Twelve of these patients were then followed for 38 to 58 months of open-label treatment (Phase II). Clinical responses were assessed with traditional indices, treadmill exercise tests, and a newly developed Performance Index (PI). During Phase I, verapamil resulted in a 50% mean reduction in the number of patients developing effort angina on treadmill exercise, a 15% mean increase in treadmill exercise time, and an 18% mean improvement in the PI. In eight of ten patients, diastolic blood pressure rises during exercise were significantly lowered during verapamil treatment in Phase I. In a limited population of patients followed for long periods of time, our data show that verapamil remains acceptably effective in the treatment of angina pectoris. Though diastolic blood pressure rises were decreased during Phase I, we could not confirm that effect over longer periods of time. Functional capacity as determined by the PI was sustained in Phase II.