Cyclosporin A has been used in conjunction with allogeneic bone-marrow transplantation in the treatment of 23 patients--21 with acute leukaemia, 1 with chronic granulocytic leukaemia, and 1 with aplastic anaemia. The drug was given twice daily from the day before transplant. At the start of the study cyclosporin prophylaxis was stopped in 3 patients within 44 days of transplantation because of non-specific rashes and/or deteriorating renal function. All 3 patients had acute graft-versus-host disease (GVHD) and died. Thereafter the drug was not stopped because of possible toxic manifestations, and 20 patients have been studied (median follow-up 7 months; maximum 13 months). 2 patients have acquired GVHD; 1 patient died of acute GVHD and 1 has chronic mild disease. 3 other patients have died, 2 of recurrent leukaemia and a third of staphylococcal pneumonia with renal failure. Of the remaining patients, 1 has recurrent leukaemia and 1 has moderately severe renal failure. Several toxic effects of cyclosporin A have been observed but they are mostly reversible and no second malignant neoplasm has developed.