Potential age-dependent changes of sirolimus metabolite patterns in pediatric renal transplant recipients remain elusive. Thirteen pediatric solid organ transplant recipients (10 kidney, one combined liver-kidney, two liver, mean age 8.0 +/- 5.0 yr) underwent a sirolimus pharmacokinetic profile in steady-state with 10 samples drawn over 12 h post-intake to calculate the AUC(0-12 h). Concentrations of sirolimus and metabolite were quantified using a validated LC-MS/MS assay and metabolite structures were identified directly in blood extracts using LC-MS/iontrap. Average sirolimus AUC(0-12 h) was 64.9 +/- 29.7 ng h/mL. Median (range) AUC(0-12 h) for each metabolite (ng h/mL) was: 12-hydroxy-sirolimus 7.6 (0.2-18.8), 46-hydroxy sirolimus 3.1 (0.0-12.4), 24-hydroxy sirolimus 4.3 (0.0-12.6), piperidine-hydroxy sirolimus 3.5 (0.0-8.3), 39-O-desmethyl sirolimus 3.6 (0.0-11.3), 16-O-desmethyl sirolimus 5.0 (0.1-9.9), and di-hydroxy sirolimus 4.3 (0.0-32.5). The metabolites reached a median total AUC(0-12 h) of 60% of that of sirolimus. The range was 2.6-136%, indicating significant variability. In all, 77.5% of the metabolites were hydroxylated, while 39-O-desmethyl sirolimus accounted for only 8.4% of the AUC(0-12 h). This is clinically relevant as 39-O-desmethyl sirolimus shows 86-127% cross-reactivity with the antibody of the widely used Abbott sirolimus immunoassay. The metabolism of sirolimus in the children included in our study differed from that reported in adults, which should be considered when monitoring sirolimus exposure immunologically.