Presently, there is little knowledge regarding cyclosporine (CsA) concentration at 2 hr post-dose (C2) monitoring in maintenance patients. This study evaluates the actual C2 range in stable renal transplant recipients (who underwent transplantation >12 months ago). In addition, we investigated whether underexposure or overexposure to CsA (assessed by C2) affects graft function (as measured by serum [S]-creatinine). All renal transplant recipients in Norway receiving CsA were asked to participate; 1447 fulfilled the criteria. Valid C2 and CsA trough concentration (C0) measurements were performed in 1032 renal transplant recipients (71%) monitored by C0. Target C0 level was 75 to 125 mumol/L. CsA levels were measured using a Cloned Enzyme Donor Immunoassay method, and all analyses were performed in the same laboratory (overall mean [+/-standard deviation] CsA C0=112+/-31 mug/L, CsA C2=697+/-211 mug/L [range 81-1580 mug/L], CsA dose [mg/day]=208+/-61, CsA dose [mg/kg/day]=2.8+/-1.1, and S-creatinine=141+/-58 mumol/L). A univariate analysis of variance showed that patients with C2 levels between 700 and 800 mug/L (n=203, S-creatinine=136+/-49 mumol/L) had significantly lower S-creatinine levels compared with patients with C2 levels greater than 950 mug/L (n=94, S-creatinine=152+/-56 mumol/L) (P<0.02). The same was true for patients with C2 levels less than 450 mug/L (n=95, S-creatinine 141+/-72 mumol/L) (P<0.05) when compared with patients with C2 levels greater than 950 mug/L. There was no significant difference in S-creatinine between patients in the low and intermediate C2 group; 666 patients had C0 levels in the therapeutic range (75-125 mumol/L). A linear regression showed a significant relation between S-creatinine and C2 for these patients (P=0.03). The corresponding relation between S-creatinine and C0 was nonsignificant (P=0.3). Monitoring of C2 in maintenance patients is a valuable tool to detect overexposure to CsA. Until results from prospective studies are available, we recommend C0 in the therapeutic range and reduction in CsA in overexposed patients, aiming at a C2 value between 700 and 800 mug/L.