Background. Cyclosporin A (CyA) is a drug with a narrow therapeutic window and highly variable pharmacokinetics. Therapeutic drug monitoring is essential and conventionally has been guided by trough levels (C-0). Recent evidence indicates that a single blood concentration measurement 2 h after CyA administration (C-2) is a more accurate predictor of drug exposure and clinical events than determination of C-0. To date, limited prospective data are available with respect to risks and benefits of C-2 monitoring in renal transplant recipients, and little experience exists with C-2 monitoring in maintenance patients. Methods. In 127 long-term renal allograft recipients, we determined C-2 levels in addition to conventional C-0 and observed clinical outcome over a period of 13.6 +/- 3.1 months. To determine the precision of monitoring, we repeatedly determined C-0 and C-2 levels in 46 stable patients without dose change. Results. Clinical outcome was excellent (patient survival 100%, graft survival 97%), with only two borderline rejections, although C-2 levels (564 +/- 186 ng/ml) were lower than recommended so far for maintenance patients. We found no significant differences in C-2 levels between patients with rejection and CyA toxicity. Receiver operating characteristic (ROC) analysis showed no prediction for risk of rejection, toxicity or infection by C-2 levels. Repeated determinations of both C-0 and C-2 levels in 46 patients revealed a high intra-patient variability. In these patients, the coefficient of variation for C-2 was only marginally better compared with C-0. Conclusions. We conclude that in maintenance patients, C-2 concentrations between 500 and 600 ng/ml are well tolerated and provide effective and safe rejection prophylaxis. Although mean C-2 levels do not seem to be helpful in identifying patients at risk for rejection, they may be useful to detect over-immunosuppression and to improve long-term allograft survival further by reducing CyA nephrotoxicity.