['Dilated cardiomyopathy is characterized by an imbalance between left ventricular performance and myocardial energy consumption. Experimental models suggest that oxidative stress resulting from increased xanthine oxidase (XO) activity contributes to this imbalance. Accordingly, we hypothesized that XO inhibition with intracoronary allopurinol improves left ventricular efficiency in patients with idiopathic dilated cardiomyopathy.', 'Patients (n=9; ejection fraction, 29+/-3%) were instrumented to assess myocardial oxygen consumption (MVO(2)), peak rate of rise of left ventricular pressure (dP/dt(max)), stroke work (SW), and efficiency (dP/dt(max)/MV O(2) and SW/MVO(2)) at baseline and after sequential infusions of intracoronary allopurinol (0.5, 1.0, and 1.5 mg/min, each for 15 minutes). Allopurinol caused a significant decrease in MVO(2) (peak effect, -16+/-5%; P<0.01; n=9) with no parallel decrease in dP/dt(max) or SW and no change in ventricular load. The net result was a substantial improvement in myocardial efficiency (peak effects: dP/dt(max)/MVO(2), 22+/-9%, n=9; SW/MVO(2), 40+/-17%, n=6; both P<0.05). These effects were apparent despite concomitant treatment with standard heart failure therapy, including ACE inhibitors and beta-blockers. XO and its parent enzyme xanthine dehydrogenase were more abundant in failing explanted human myocardium on immunoblot.'] Does allopurinol improve myocardial efficiency in patients with idiopathic dilated cardiomyopathy?