To determine the clinical impact of conversion from C0 to C2 Neoral monitoring, we conducted a 6-month prospective study in 62 stable renal transplant recipients. Neoral was given alone (19%), with steroids (31%), combined with azathioprine (Aza) or mycophenolate mofetil (MMF) (50%). C0 and C2 target ranges were, respectively, 130-190 and 700-900 ng/mL. Neoral dosages were adjusted according to the C2 range. At baseline, mean C0 and C2 were 157 and 762 ng/mL. After 6 months C0 was 173 ng/mL (p<0.02) and C2 was 804 ng/mL (ns). Although the mean Neoral dose at 6 months was unchanged from baseline, the dose was reduced in 24 patients from 3.6+/-1.2 to 3.0+/-0.9 mg/kg/day, with a mean reduction in serum creatinine (Cr) from 1.4+/-0.4 to 1.3+/-0.3 mg/dL (p<0.001), stable in 8 patients and increased in 30 patients from 3.3+/-1.0 to 3.8+/-1.2 mg/kg/day with no change in serum Cr. Serum transaminases and blood pressure (BP) were unchanged in the three groups. C0 and C2 showed a positive correlation, but with a large dispersion of values (r2=0.14, p<0.001). Overall concordance between the C0 and C2 ranges was 49%. Therefore, in stable transplant patients C0 cannot be considered a C2 surrogate. The conversion from C0 to C2 led to a Neoral dose reduction in approximately 40% of patients with significant improvement in renal function. Most of the remaining patients required an increased dose; however, without an increased incidence of cyclosporin-induced side-effects.