Data from the North American Pediatric Renal Transplant Cooperative Study were analyzed to determine the incidence and possible causes of graft thrombosis in pediatric renal transplant recipients. Between January 1987, and November 1989, 1045 renal transplants in recipients less than 18 years of age were registered in the study, including 484 living-related donor and 561 cadaver donor transplants. There were 213 graft failures (67 LRD, 146 CAD), and of these 27 were caused by thrombosis (8 LRD, 19 CAD). Thrombosis occurred in 2.6% of all transplants and accounted for 22.5% (27/120) of all graft failures that occurred in the first 60 days following transplantation. Among the LRD recipients, there were 24 graft failures in those less than 6 years of age, and 7 of these were due to thrombosis, compared to 1 thrombosis in 43 graft losses in recipients greater than 6 years (P less than 0.01). In recipients less than 6 years old, the thrombosis rate for those who received transplants without prior dialysis was 4/32 (12.5%) versus 3/109 (2.8%) with prior dialysis. Among the CAD recipients, age of the recipient did not influence graft thrombosis. Donor age, however, was strongly associated with the risk of thrombosis, as was cold storage time. Donor age and cold storage time were not independently distributed within the population, with longer cold storage times required for younger donors. Both factors, however, independently affected outcome. Other factors, including prior nephrectomy, prior transplant, center size, and use of cyclosporine were not associated with increased risk of thrombosis in LRD or CAD recipients. We conclude that graft thrombosis is an important cause of renal graft loss in children. In LRD transplants the risk of graft thrombosis is increased in recipients less than 6 years old, and preliminary data suggest that the lack of prior dialysis may be associated with thrombotic risk in these patients. CAD transplant recipients who receive grafts from young donors, particularly those with long cold storage time, are at increased risk for graft failure due to thrombosis.