Eighteen months of experience with cyclosporin A (Cy A) in allogeneic bone marrow transplantation for severe aplastic anemia (SAA) is reported. Twenty-three patients treated with Cy A for postgraft immunosuppression are described and compared with 14 similar patients with SAA in whom methotrexate (MTX) was used. The early results are encouraging with 73% survival in the Cy A group compared with 43% in the MTX group. The improvement is partly attributable to the low incidence of graft failure. Graft-vs.-host disease (GVHD) remains a problem with an overall incidence of 70% in Cy A-treated aplastic patients, although mortality has been only 14%. Toxicity attributable to Cy A has so far been acceptable; nephrotoxicity is usually mild and reversible. However, 3 aplastic patients have developed clinically significant renal impairment while receiving both Cy A and aminoglycoside antibiotics.