Cyclosporin A was given to five patients with acute leukæmia in whom graft-versus-host disease (G.V.H.D.) had developed after bone-marrow transplantation from sibling donors. In all instances the acute erythematous skin reaction of G.V.H.D. resolved within two days, but four of the five patients died. Cyclosporin A in high doses produced anorexia, nausea, and a reversible rise in blood-urea. The four patients who died all had liver damage, but the histological changes varied. Cyclosporin A modifies the acute skin reaction of G.V.H.D. In the management of liver and gut G.V.H.D., and in prophylaxis of G.V.H.D., its role needs to be determined.