It is widely recognized that the area under the concentration-time curve (AUC) correlates with the clinical effects of cyclosporine MEPC (Neoral) therapy and adverse reactions to it. In this study, we investigated which single blood-level measurement had correlated best with AUC in interstitial pneumonia therapy. Neoral was prescribed in ten cases of interstitial pneumonia. We measured the concentration during the 4 hours following its administration and calculated AUC0-4 (AUC 0-4 hr). The concentration after 2 hours of administration (C2) correlated best with AUC (r2 = 0.71). This result suggests that C2 monitoring would be a better predictor for AUC than C0 (trough level) monitoring in interstitial pneumonia therapy.