Cyclosporin A (CsA) has been used most widely as an immunosuppressive agent for preventing graft-versus-host disease (GVHD). To explore the risk factors including CsA blood levels for grades II-IV acute GVHD, we retrospectively analyzed the data of patients who underwent allogeneic hematopoietic stem cell transplantation in our hospital between March 1989 and July 2001. Seventy-three patients ( 47 males and 26 females) received CsA and short-term methotrexate for GVHD prophylaxis. CsA 1.5 mg/kg was administered as a 3-h infusion twice daily from day -1 until the patient recovered from the toxic gastrointestinal complication. Methotrexate was given at a dose of 15 mg/m(2) on day 1 and 10 mg/m2 on days 3, 6 and 11. Grades II-IV acute GVHD occurred in 18 patients ( 24.7%). Multivariate Cox regression analysis revealed that higher C-5 ( the whole-blood CsA concentration at 5 h after the start of infusion) before the onset of acute GVHD reduced the onset of grades II-IV acute GVHD with a hazard ratio of 0.994 (95% confidence interval 0.989-0.999) for every increase of 1 ng/ml. Our data indicate that inadequate exposures of CsA can be a vital risk for developing acute GVHD. From our results, we consider that precise monitoring of CsA concentrations and adjustment of CsA dose using the concentration may be effective to prevent the onset of severe acute GVHD. To confirm this finding, further prospective study will be needed.