Twice-daily administration of cyclosporine A (CyA) has often been used for prophylaxis of acute graft versus host disease in allogeneic hematopoietic stem cell transplantation (allo-HSCT). But there have not been any reports that calculated the conversion ratio of the switch from twice-daily intravenous infusion to oral administration of CyA in adult patients. To establish the conversion ratio and the best strategy of twice-daily administration of CyA, the authors investigated the serial changes in the blood CyA concentration during the switch from twice-daily intravenous infusion to oral administration while maintaining high-peak concentration (over 1000 ng/mL) and calculated the bioavailability of Neoral, a micro emulsion cyclosporine, in 11 patients. All the patients underwent allo-HSCT with graft versus host disease prophylaxis consisting of CyA at a high-peak concentration of twice-daily infusion with short-term methotrexate and oral administration. Neoral was started at an oral dose, 2 times daily, at twice the latest dose of intravenous dose according to the bioavailability of healthy volunteers. Micafungin, a mild inhibitor of CYP3A4, was administered for prophylaxis against fungal infection. The total area under the concentration-time curve during oral administration (AUCpo) was nearly the same as AUC during intravenous infusion (AUCiv) (mean ± SD, 7206 ± 1557 ng·h·mL and 7783 ± 897.7 ng·h·mL, respectively). The bioavailability of Neoral, defined as F = AUCpo × DOSEiv/AUCiv × DOSEpo was 0.58 ± 0.15 (mean ± SD, range: 0.41-0.94). When patients were switched from twice-daily infusion to oral administration, the dose conversion ratio of 1:2 seemed to be appropriate. At that time, the target trough level of Neoral was nearly the same as that of the infusion.