89 patients with aplastic anemia or acute leukemia treated by either syngeneic (13 patients) or allogeneic (76 patients) marrow transplantation who had survived for more than 6 mo were surveyed to determine the incidence of late infections. Most patients (72%) had either no infection or very few late infections, and only a minority (28%) had three or more infections. Eight patients (9%) died from infection. Bacterial infections of lung, respiratory tract, skin, and blood represented more than half of the 152 infections encountered. Gram-positive cocci represented one-third of the documented infecting organisms. Varicella-zoster (VZ) infections were seen in 22% of all patients. Each affected patient had only a single episode of VZ infection. Fungal infections and interstitial pneumonia were uncommon. By the use of a proportional hazards regression model the authors attempted to identify clinical and immunologic factors predictive of and predisposing to late infections. Only one factor was significantly associated with late non-VZ infections: chronic graft-versus-host disease (C-GVHD). Other factors analyzed, e.g., underlying disease, conditioning regimen, type of transplant, inability to respond to cutaneous dinitrochlorobenzene (DNCB), age, and sex, did not show significant associations. In contrast, VZ infections were not associated with C-GVHD. Negative DNCB skin test reactivity appeared to be the only factor correlating with VZ infections. The recurrent (and occasionally fatal) infections seen in a minority of long-term survivors after syngeneic and allogeneic marrow transplantation are most likely the result of deficits of humoral and cellular immunity, which in turn are most often associated with C-GVHD.