To determine the incidence of secondary cancers after bone marrow transplantation, we reviewed the records of all patients at our center who received allogeneic, syngeneic, or autologous transplants for leukemia (n = 1926) or aplastic anemia (n = 320). Thirty-five patients were given a diagnosis of secondary cancer between 1.5 months and 13.9 years (median, 1.0 year) after transplantation. Sixteen patients had non-Hodgkin's lymphomas, 6 had leukemias, and 13 had solid tumors (including 3 each with glioblastoma, melanoma, and squamous-cell carcinoma). There were 1.2 secondary cancers per 100 exposure-years during the first year after transplantation (95 percent confidence interval, 0.7 to 2.0). The rate declined to 0.4 (95 percent confidence interval, 0.2 to 0.7) after one year. The age-adjusted incidence of secondary cancer was 6.69 times higher than that of primary cancer in the general population. In a multivariate model, the predictors (and relative risks) of any type of secondary cancer were acute graft-versus-host disease treated with either antithymocyte globulin (relative risk, 4.2) or an anti-CD3 monoclonal antibody (13.6) and total-body irradiation (3.9). Two additional factors were associated with secondary non-Hodgkin's lymphomas: T-lymphocyte depletion of donor marrow (12.4) and HLA mismatch (3.8). We conclude that recipients of bone marrow transplantation have a low but significant risk of a secondary cancer, particularly non-Hodgkin's lymphoma.