Graft coronary artery vasculopathy is the main cause of late morbidity and mortality in cardiac allograft recipients. A high plasma homocysteine (hcy) concentration is now generally accepted as a risk factor for coronary arteriosclerosis, but little information exists for the pediatric age group. We therefore explored the potential role of hcy and antioxidants in 31 pediatric allograft recipients. We found hcy concentrations to be significantly higher in recipients than in control. Hcy continued to rise within the first 2 postoperative years. Vitamin A and E concentrations were significantly lower in transplant patients. Hyperhomocysteinemia was significantly more common in patients with complications than in those without. Our findings suggest that pediatric allograft recipients experience oxidant stress, as highlighted by the high plasma levels of Hcy and the low concentrations of vitamins A and E. Nutritional supplementation may be indicated to lower plasma hcy and to reduce oxidant stress.