L-propionylcarnitine, a naturally occurring derivative of L-carnitine, essential for mitochondrial fatty acid transport and high-energy phosphate exchange, acutely reduces myocardial ischaemia and improves ischaemia-induced cardiac dysfunction following intravenous administration. This randomized, crossover study was designed to compare the long-term anti-ischaemic effects of oral L-propionylcarnitine with diltiazem in patients with stable, exercise-induced angina. After a 2-week washout phase of anti-anginal medication and a 2-week single-blind placebo period, 46 patients were included in the study, 23 of whom received 1500 mg L-propionylcarnitine daily for 6 weeks, and 23 diltiazem (180 mg daily for 3 weeks, followed by 360 mg daily for 3 weeks), crossing over to the other treatment after a 1-week washout period. Three patients on L-propionylcarnitine and two on diltiazem discontinued. Both treatments resulted in comparable exercise duration (582 +/- 35 s and 588 +/- 33 s, mean +/- SEM), time to 0.1 mV ST depression (436 +/- 38 s and 465 +/- 36 s), and increase in time to 0.1 mV ST depression from baseline (20% and 28%), L-propionylcarnitine and diltiazem, respectively. Diltiazem decreased the rate-pressure product at rest and exercise, L-propionylcarnitine did not. Both compounds significantly reduced ST depression at maximal exercise [23% (L-propionylcarnitine) vs 35% (diltiazem), P < 0.05 diltiazem vs L-propionylcarnitine]. Diltiazem increased the time to onset of angina by 22%. In contrast, no significant changes occurred with L-propionylcarnitine. During the study, anginal attacks were reduced by 70% and 57%, and nitroglycerin consumption decreased by 57% and 70%, L-propionylcarnitine and diltiazem, respectively. Thus, both L-propionylcarnitine and (high-dose) diltiazem result in anti-ischaemic effects and decrease anginal attacks in daily life. Although the effect of diltiazem on exercise-induced ischaemia appears more pronounced than that of L-propionylcarnitine, this novel metabolic approach to ischaemia warrants further development.