Survivors of allogeneic marrow transplants are immunodeficient for at least 1 year after grafting. Multiple defects of immunity have been found; however, it is not known which defect primarily accounts for the high infectious morbidity of these patients. Twenty-nine allograft recipients who were in complete remission of the original disease were examined for the following parameters of immunity at 1 year after transplant: infection score (gauging the number and severity of infections within the 6 months prior to the annual exam), serum total IgM, IgG, and IgA, anti-Haemophilus influenzae IgG, anti-Streptococcus pneumoniae IgG, skin test reactivity, and the blood counts of B cells, CD4+ T cells, CD8+ T cells, and their subsets. THe only parameter inversely correlated with the infection score was CD4+ T cell count (P = 0.005 in univariable analysis, P = 0.06 in multivariable analysis). We conclude that infectious morbidity of long-term transplant survivors is related to the reconstitution of CD4+ T cells.