The relationship between CsA levels and area under the curve (AUC) in allo-SCT recipients, and the effect of age, concomitant use of steroid and MDR-1 polymorphism on this relationship remain largely unexplored. Steady-state CsA blood concentrations at time 0 (C0), 1 (C1), 1.5 (C1.5), 2 (C2), 3 (C3), 4 (C4), 6 (C6), 8 (C8) and 12 (C12) h post oral CsA dose were taken from 27 consenting allo-SCT recipients (receiving myeloablative or nonmyeloablative conditioning) at D 15 -D 25 (all participants) and D 40 -D 80 (participants with myeloablative conditioning). The CsA AUC 0À4h , AUC 0À8h and AUC 0À12h were determined using trapezoidal rule, and the relationships between AUCs and CsA concentrations at various time points were examined. Poor correlation was observed between C0 and AUC 0À4h (r 2 ¼ 0.15), AUC 0À8h (r 2 ¼ 0.21) and AUC 0À12h (r 2 ¼ 0.53). C2 was better correlated with AUC 0À4h (r 2 ¼ 0.88), AUC 0À8h (r 2 ¼ 0.76) and AUC 0À12h (r 2 ¼ 0.83). The aforementioned factors did not influence the observed relationship. CsA levels taken at 2 h post oral CsA administration may represent the optimal time point for monitoring the biological effects of calcineurin inhibitors in allo-SCT recipients.