Of 68 consecutive allogeneic bone marrow transplant patients, 53 survived more than three months after transplantation. Twenty-two (42%) developed chronic graft-versus-host disease (GVHD). Chronic GVHD was more common among patients who had previously experienced cytomegalovirus (CMV) infection (20/36, 56%) than among those without signs of active CMV infection (2/17, 12%) (P less than 0.01). The CMV infections preceded the development of chronic GVHD by a median of 128 days (range 23-322 days). Children below 14 years of age who had had CMV infection developed chronic GVHD as often as older patients (8/14 vs. 12/22). CMV infection may pave the way for chronic GVHD.