Summary: Trough levels (TL) of cyclosporine (CS) measured in serum by the polyclonal radioimmunoassay (SR) are useful for dissecting the etiology of clinical events, but they are a poor guide to dosage adjustments. In renal transplant patients immunosuppressed by low doses of prednisone and CS given orally, once-a-day TL (24-h) monitoring was replaced by area under the curve (AUC) monitoring, i.e., measuring the area under the concentration (SR)time curve from seven blood samples (0, 2, 4, 6, 10, 14, and 24 h) at clinical steady state, which was reached on the 3rd day after a change in the oral dose rate. The therapeutic target was an average concentration at steady state (Css av) of 200 ng/ml during the first 6 months after transplantation and 150 ng/ml thereafter. TheCss avwas calculated by dividing the AUC by the dosing interval (24 h). Two findings demonstrated the superiority of AUC monitoring over TL monitoring. First, in 71 paired observations AUC but not TL was significantly correlated with the dose expressed as total mg (r= 0.381, p = 0.001) or mg/kg body weight (r = 0.538, p = 0.0001). Second, after adjusting (n = 26) the oral dose rate (to achieve the therapeutic target) the absolute error (i.e., deviation from the target) in the AUC observation (15%) was significantly (p = 0.0005) smaller than in the TL observation (36%). Monitoring AUC at clinical steady state reduced the number of dosage adjustments by a factor of 3.