A group of 46 bone marrow transplant (BMT) recipients were studied by lymphocyte stimulation with cytomegalovirus (CMV) antigen before, and repeatedly in the first year after, BMT. Of these, 25 patients developed CMV infection and 68% of these got a positive lymphocyte response to CMV. The recipients with an early response (up to 3 months) to CMV antigen after the CMV infection were less prone to develop chronic graft-versus-host disease (GVHD) than those who responded later or not at all (P = 0.002). After the CMV infection, the increased lymphocyte CMV reactivity remained in recipients without chronic GVHD, but recipients with chronic GVHD usually lost their reactivity. The results suggest that it may be possible to predict patients who are not going to develop chronic GVHD by studying lymphocyte responses to CMV infections.