A double-blind phase II study of propionyl-L-carnitine (CAS 17298-37-2) versus placebo was carried out on a group of 60 patients with mild to moderate (II and III NYHA class) congestive heart failure. The group was made up of men and women aged between 48 and 73 years in chronic treatment with digitalis and diuretics for at least 3 months and who still displayed symptoms. Thirty of these patients were chosen randomly and for 180 days, 500 mg of propionyl-L-carnitine was orally administered, 3 times a day in addition to their usual treatment. At basal conditions and after 30, 90 and 180 days the maximum exercise time was evaluated using an exercise tolerance test performed on an ergometer bicycle and the left ventricular ejection fraction was tested by means of bidimensional echocardiography. After one month of treatment, the patients treated with propionyl-L-carnitine, compared to the control group, showed significant increases in the values of both tests, increases which became even more evident after 90 and 180 days. At the stated times the increases in the maximum exercise time were 16.4%, 22.9%, and 25.9%, respectively. The ventricular ejection fraction increased by 8.4%, 11.6% and 13.6%, respectively. On the basis of these results, having studied the particular mechanism of action of propionyl-L-carnitine the authors conclude that it represents a drug of undoubted therapeutic interest in patients with congestive heart failure, in whom it could be efficaciously administered along with a standard pharmacological therapy.