Clinical evolution and cyclosporine (CsA) monitoring of 65 transplanted patients (55 kidneys, and 10 kidneys and pancreases) treated with CsA were analyzed retrospectively (45 patients) and prospectively (34 patients). Our results showed the following: (1) nephrotoxicity is not uncommon even with low trough plasma levels of CsA; (2) the T6 value of a CsA pharmacokinetic plasma curve (6 hr after oral drug administration) is a valid expression of a full pharmacokinetic study; (3) when T6 was used prospectively as a monitoring tool and dose adjustments made disregarding concomitant serum creatinine levels, the latter decreased when CsA dose adjustments were made to correct toxic (greater than 350 ng/ml) or subtherapeutic (less than 100 ng/ml) T6, P less than 0.01. At present, serum creatinine for all our patients is 180.2 +/- 8 mumol/L, and no patient has needed to be switched to conventional treatment. The validity of trough plasma levels in patients under CsA oral administration once or twice a day seems questionable, and T6 proved to be more useful. Thus nephrotoxicity and CsA undertreatment may be avoided. This new monitoring tool (T6) will allow the utilization of lower doses of CsA and thus contribute to improved long-term graft function.