In solid organ transplantation, CYA dosing is based on the area under the concentration vs time curve (AUC(inf)). This study aimed to develop a guideline for the initial i.v. CYA dose for pediatric hematopoietic SCT (HSCT) patients to achieve the target AUC(inf) recommended in solid organ transplantation. Whole-blood CYA concentrations were determined in 24 patients (0.5-16.9 years) after the first i.v. dose given over 2 h, 1 day before HSCT. The i.v. CYA dose predicted to achieve an AUC(inf) of 4200 microg x h/l was calculated for each patient and expressed as a function of each patient's actual weight and body surface area (BSA). In patients <or=9 and >9 years of age, the mean i.v. CYA dose predicted to achieve the target AUC was 2.6+/-0.94 and 2.1+/-1.21 mg/kg, respectively. When these doses were expressed in terms of BSA, the mean dose was 65+/-23.1 and 68+/-35.0 mg/m(2) in children <or=9 and >9 years of age, respectively. In children 0.5-17 years of age undergoing HSCT, we recommend an initial i.v. CYA dose of 65 mg/m(2) infused over 2 h to achieve an AUC(inf) of approximately 4200 microg x h/l.