This study evaluates our institution's target trough cyclosporine (CSA) concentrations as predictors of severe acute graft versus host disease (aGvHD) in children receiving either matched related or unrelated hematopoietic stem cell transplantation (HSCT). The outcomes of 87 consecutive children who underwent allogeneic HSCT and received CSA and methotrexate as prophylaxis against aGvHD between October 1, 1999 and September 30, 2002 were retrospectively evaluated. The proportion of time that each patient maintained a whole blood CSA concentration within or above the initial target range (105-155 ng/mL or 155-210 ng/mL) was calculated for each of the following time periods: in each week after HSCT from day 0 to +28; in the week preceding engraftment; and in the week preceding the onset of aGvHD. Patients were prospectively evaluated twice weekly for the presence and severity of aGvHD by senior attending physicians. The relationship between potential predictors and the development of severe aGvHD was examined using univariate logistic regression. The main variables of interest were the proportion of time that therapeutic or supratherapeutic CSA concentrations were maintained; median CSA concentrations; the number of methotrexate doses received; and the use of folinic acid rescue. Mean follow-up time was 3.0+/-1.9 years among children who survived beyond day +100. Three variables were significantly associated with the development of severe aGvHD on univariate analysis: initial CSA target concentration [odds ratio (OR), 0.24; P=0.03], proportion of time the target CSA concentration was achieved during the second week after transplant (OR, 0.16; P=0.02), and proportion of time the target CSA concentration was achieved during the week before engraftment (OR, 0.22; P=0.0489). Multivariable analysis demonstrated an inverse relationship between the median CSA concentration during the week before engraftment and the development of severe aGvHD (OR, 0.99; P=0.045). These results suggest that achievement of our CSA target concentrations is important to aGvHD outcomes.