Nearly one-half of marrow transplant recipients who survive at least 6 months develop varicella-zoster virus (VZV) infection. Of 92 cases studied, 82 occurred within the first 12 months after transplant. Only one patient had recurrent infection. Seventy-seven patients had herpes zoster, 22 with subsequent cutaneous dissemination, and 15 had varicella. The overall mortality rate was 8%, and all deaths occurred within 9 months of transplant. Twenty-six of 32 patients studied had significant rises in VZV antibody during recovery. Among patients with acute leukemia, those with syngeneic transplants had a significantly lower incidence of VZV infection than those with allogeneic transplants. Incidence was slightly, but not significantly, decreased among patients with aplastic anemia. In contrast to other infections, the incidence of VZV infection was not influenced by graft-versus-host disease or predicted by the results of dinitrochlorobenzene skin testing.