The effect of antithymocyte globulin (ATG) on quantitative immune recovery and graft-versus-host disease (GVHD) after partially T-cell-depleted bone marrow transplantation was analyzed in 59 and 32 recipients of grafts from matched related donors and matched unrelated donors (MUDs), respectively. The conditioning regimen was similar in all patients, except for ATG which was given only to MUD recipients. Thirteen MUD patients were treated with high-dose (20 mg/kg) ATG and 19 with low-dose (8 mg/kg) ATG. During the posttransplant period, CD3+, CD4+, and CD8+ T-cell numbers and the incidence of acute and chronic GVHD were significantly lower in MUD recipients compared with matched related donor recipients. MUD recipients treated with high-dose ATG showed the worst T-cell and subsets recovery. These data indicate that ATG, often used as part of conditioning regimens in recipients of T-cell-depleted grafts from MUDs, contributes to the severe and prolonged T-cell deficiency that is typical of these patients. On the other hand, it effectively reduces the incidence and severity of GVHD.