Acute and chronic L-carnitine application exerts protective effects in a number of cardiac diseases. These favourable effects are attributed to improvements of the energy metabolism and have been found both in animal experiments and in man. In order to investigate the effect of long-time oral L-carnitine substitution on physical performance, 41 patients suffering from class NYHA II or III cardiac insufficiency were recruited for a clinical study. Following the double-blind, randomized, placebo-controlled design of the study, 20 patients were given 3 x 1 g L-carnitine daily for 120 days whereas the control group (21 patients) received placebo. Bicycle ergometer tests were used to determine maximum performance, systolic and diastolic blood pressure, heart rate, and ST changes. Four series of tests were carried out: on day 0 (before the first substrate application), on the 60th and the 120th day (during L-carnitine or placebo application), and on the 180th day (60 days after the end of substitution). A significant improvement in performance (significantly higher maximum performance during bicycle ergometry) could be found within the carnitine group on the 60th and 120th day of L-carnitine application; and haemodynamical parameters showed a tendency to improve, too. These effects, which were attributed to L-carnitine, could be detected even 60 days after the end of substitution. No corresponding changes were found in the placebo group. The findings presented in this paper support suggestions of other authors that L-carnitine in combination with the usual medication (digitalis, beta-blockers, calcium antagonists, nitrates) improves performance and effort tolerance in patients with cardiac insufficiency. Moreover, the findings suggest a favourable long-term effect, which lasts beyond the actual L-carnitine application, on the performance of patients with advanced cardiac insufficiency.