A total of 21 patients with severe aplastic anemia (SAA) underwent marrow transplantation from HLA-identical siblings following a standard conditioning regimen with cyclophosphamide (50 mg/kg/day × 4 days) and horse antithymocyte globulin (30 mg/kg/day × 3 days). Post-grafting immunosuppression consisted of a short course of methotrexate (MTX) combined with cyclosporine (CSP). The transplant protocol tested the hypothesis that the incidence of chronic GvHD could be reduced by limiting the marrow grafts to ⩽2.5 × 108 nucleated marrow cells/kg. None of the patients rejected the graft, all had sustained engraftment and all are surviving at a median of 4 (range 1-8) years after transplantation. Chronic GvHD developed in 16% of patients given ⩽2.5 × 108 nucleated marrow cells/kg. Post-grafting immunosuppression has been discontinued in 20 of the 21 patients. In conclusion, limiting the number of transplanted marrow cells may have resulted in minimal improvement in the incidence and severity of chronic GvHD.