Organ transplant recipients receive immunosuppressive drugs to prevent graft rejection. This treatment has been associated with higher rates of non-Hodgkin lymphoma (NHL) than in the general population. We assessed the incidence of NHL in a multicentre study of 45,141 kidney transplant patients and 7634 heart transplant recipients. The NHL rate was especially high during the first post-transplant year among both kidney transplant recipients (101 cases vs 2.7 expected in general population; 224 per 10(5)) and heart transplant recipients (93 vs 0.6 expected; 1218 per 10(5)). The incidence was lower in subsequent years (43 and 371 per 10(5) in kidney and heart transplant recipients). During the first year the NHL incidence was higher in North America than in Europe (relative risk 2.12 [95% CI 1.55-2.89]). There were also significant increases in risk for patients who received rejection prophylaxis with antilymphocyte antibodies (1.80 [1.31-2.46]) and in those who received both cyclosporin and azathioprine rather than another immunosuppressive combination (1.47 [1.03-2.08]). This study quantified the risk of NHL after kidney or heart transplantation. It suggests that the risk of NHL is related to the aggressiveness of the immunosuppressive regimen.