We performed an intravenous cyclosporine (CsA) drip infusion method for 3 hours and C3 monitoring in pediatric hematopoietic stem cell transplantation and examined the internal change and monitoring method of CsA. A total of six cases comprised five cord blood transplantations and one related allogeneic bone marrow transplantation. We started CsA 1.5 mg/kg at the day before transplantation by intravenous drip infusion (twice a day) for three hours. We controlled the dose so that the optimal peak value of C3 reached 800-1000 ng/ml. We recognized the C3 peak occurred three hours after initiation of infusion, and the blood CsA concentration was restored to the baseline value (C0) to (C12) 12 hours after that. We found a strong correlation between the C3 value (r = 0.90), and AUC(0-12). Two patients had grade II acute graft-versus-host disease (GVHD), but one needed no treatment, and the other recovered with short-term dosage of prednisolone. Apart from these instances, no serious complication occurred. In pediatric hematopoietic stem cell transplantation, it seems that regulation of the appropriate blood CsA concentration is enabled by using C3 monitoring at around 3 hours after commencing the intravenous drip infusion method for CsA.