In adult leukemic marrow recipients of HLA identical sibling marrow, 23 patients were randomized to T cell depletion and 25 received cyclosporin (CSA) and four doses of methotrexate (MTX) to prevent graft-versus-host disease (GVHD). Anti-CD8 and anti-CD6 antibodies plus complement depleted 95.3 +/- 5.8% (mean +/- SE) CD3 cells. All patients engrafted except one who died early. Patients receiving T cell-depleted marrow had a faster time to 0.2 x 10(9) WBC/l (p less than 0.001), but required more erythrocyte units (p = 0.03). Platelet transfusions, infections and time in hospital did not differ. The incidence of grade II-III acute GVHD was 23% following T cell depletion and 12% for those receiving CSA + MTX. Chronic GVHD occurred in 51% and 23% in the two groups, respectively (p = 0.06). Recipients of T cell-depleted marrow who developed grade I-III acute GVHD received more T cells compared to those without acute GVHD (p = 0.02). The major cause of death in both groups was relapse, the cumulative incidence of which, at 4 years, was 39% in the recipients of T cell-depleted marrow and 54% in the CSA + MTX group. The 3-year actuarial leukemia-free survival was 42% and 44% in the two respective groups.