This study evaluates propranolol's effect on symptoms, arrhythmias, and exercise tolerance in 16 patients with mitral valve prolapse. Three patients (19 per cent) experienced symptomatic deterioration with propranolol therapy, seven (44 per cent) were unchanged, and six (37 per cent) noted an over-all symptomatic improvement, primarily due to a reduction in palpitation. Symptomatic improvement continues in these six patients an average of 12.5 months after beginning propranolol therapy. Treatment with propranolol alleviated chest pain in only two of eight patients and it did not improve the ability to perform treadmill exercise. Fatigue did not improve, and in three patients appeared for the first time during propranolol therapy. Premature ventricular contractions were reduced by at least 75 per cent in five of nin patients (56 per cent), and paroxysmal ventricular tachycardia was eliminated in three of four patients. We conclude that propranolol is not uniformly effective in patients with mitral vale prolapse. A trial of propranolol may be instituted fro patients with mitral valve prolapse who have severe symptoms and/or arrhythmias, but the drug should only be continued in those who demonstrate clinical and/or antiarrhythmic response.