The feasibility and toxicity of six-week continuous intravenous 3 mg/kg/day cyclosporine (CsA) treatment in conjunction with a short course of methotrexate (MTX) was studied in 69 consecutive patients after HLA genotypically identical bone marrow transplantation. In light of the uncertain efficacy of prolonged oral CsA immunoprophylaxis in preventing de novo chronic graft-versus-host disease (GVHD). CsA treatment was terminated three months after BMT. Sixty-one (88%) patients received the full intravenous regimen and no patient exclusions were necessary due to intolerable adverse effects. Weekly median blood CsA concentrations ranged between 820 ng/ml in the first and 648 ng/ml in the sixth week of treatment. No significant correlation existed between blood CsA concentrations and CsA dosages. Major adverse effects of the regimen included hypertension in 36%, acute nephrotoxicity in 36%, acute hepatotoxicity in 41%, and central nervous system toxicity in 4% of the patients. Since hepatotoxicity occurred predominantly in the early posttransplant period (median onset day 9), the relatively high incidence of this untoward effect might have been additionally caused by MTX and/or the preparative regimen. Blood CsA concentrations and CsA dosages did not significantly correlate with serum creatinine or total and conjugated bilirubin levels. In addition, blood CsA and serum creatinine levels did not differ between hypertensive and normotensive patients. Acute GVHD developed in 16% of the patients. Median CsA doses and blood CsA concentrations were identical for each week after BMT for patients contracting acute GVHD as compared with those without acute GVHD. In 55 patients surviving without acute or secondary chronic GVHD, the cumulative probability of de novo chronic GVHD after termination of CsA treatment was 13%. In conclusion, this regimen was tolerable and provided constant blood CsA concentrations for six posttransplant weeks that were not adversely influenced by the development of acute GVHD. Restriction of CsA treatment to the first three months after BMT appeared not to increase the risk of de novo chronic GVHD, which challenges regimens employing oral CsA immunoprophylaxis for 6-12 months after BMT.