To determine whether dietary antioxidant supplementation can reduce platelet reactivity in heart transplant recipients, 20 patients were prospectively randomized to receive either 500 IU vitamin E orally per day in the form of acetate for 2 months or no vitamin E. Blood creatinine (P = 0.01) and lymphocyte count (P = 0.009) significantly decreased only in supplemented patients, whereas the cyclosporine blood level was not modified. Platelet aggregation was stable in control patients but significantly decreased in supplemented patients in response to either thrombin (from 8.3 +/- 0.9% of maximum aggregation to 3.7 +/- 0.7, P = 0.001) or ADP (secondary wave: from 44.7 +/- 5.9% to 33.2 +/- 7.0, P = 0.02). Thus antioxidant supplementation tended to improve immunosuppression (by reducing lymphocyte count), to reduce cyclosporine nephrotoxicity, and to decrease the high thrombotic risk associated with heart transplantation.