We used the Collaborative Transplant Study database to analyze the incidence, risk, and impact of malignant lymphomas in approximately 200,000 organ transplant recipients. Over a 10-year period, the risk in renal transplant recipients was 11.8-fold higher than that in a matched nontransplanted population (p<0.0001). The majority of lymphomas were diagnosed after the first post-transplant year. Heart-lung transplants showed the highest relative risk (RR 239.5) among different types of organ transplants. In kidney recipients, immunosuppression with cyclosporine did not confer added risk compared with azathioprine/steroid treatment, whereas treatment with FK506 increased the risk approximately twofold. Induction therapy with OKT3 or ATG, but not with anti-IL2 receptor antibodies, increased the risk of lymphoma during the first year. Antirejection therapy with OKT3 or ATG also increased the risk. First-year mortality in renal and heart transplant patients with lymphoma was approximately 40% and 50%, respectively, and showed no improvement in recent years. A pattern of preferential localization to the vicinity of the transplant was noted, and the prognosis of the patient was related to localization. This study highlights the continuing risk for lymphoma with time post-transplantation, the contribution of immunosuppression to increased risk, and continuing poor outcomes in patients with post-transplant lymphoma.