The existence of an immunologic antileukemia reaction associated with allogeneic bone marrow transplantation (BMT) is well established. However, a similar graft-versus-tumor effect against lymphomas has not been demonstrated. We analyzed the results of BMT in 118 consecutive patients with relapsed Hodgkin's disease or aggressive non-Hodgkin's lymphoma. The 38 patients less than 50 years of age with HLA-matched donors had allogenic marrow transplants, and the other 80 patients received purged autologous grafts. The median age was 26 years in both the allogeneic and the autologous graft recipients. The patient's response to conventional salvage therapy before transplant was the only factor that influenced the event-free survival after BMT (P less than .001). Both the patient's response to salvage therapy before BMT (P less than .001) and the type of graft (P = .02) significantly influenced the probability of relapse after BMT. The actuarial probability of relapse in patients who responded to conventional salvage therapy before BMT was only 18% after allogenic BMT compared with 46% after autologous BMT. However, the actuarial probability of event-free survival at 4 years was the same, 47% versus 41%, for patients with responsive lymphomas who received allogeneic and autologous transplants, respectively (P = .8). The beneficial antitumor effect of allogeneic BMT was offset by its higher transplant-related mortality (P = .01), largely resulting from graft-versus-host disease. Allogeneic BMT appears to induce a clinically significant graft-versus-lymphoma effect. The magnitude of this effect is similar to that reported against leukemias.