Idebenone (45 mg twice daily) was administered to 7 patients with moderate renal impairment (creatinine clearance 21-40 ml/min) for 10 days. Standard pharmacokinetic parameters were computed on day 1 (single administration) and on day 10. On day 1 the mean of the maximum plasma concentration values (C(max)) was 364 ng/ml (standard deviation (S.D.) 100); time to C(max) (t(max)) was in the range of 1-2 h for 6 patients and 12 h for the remaining patient: the mean was 3 h (S.D. 3.99); the mean area under the plasma concentration vs. time curve (AUC) was 3005 ng h/ml (S.D. 1152). On day 10 the mean C(max) was 531 ng/ml (S.D. 355.3), the mean t(max) was 0.07 h (S.D. 0.19), the mean AUC was 3167 ng/ml (S.D. 2944) and the mean elimination half-life (t(1/2)) was 4.9 h (S.D. 1.1). Idebenone metabolites (QS-4, QS-6 and QS-10) showed a kinetic profile similar to the parent compound, with pharmacokinetic parameters comparable to idebenone for QS-4 and lower than idebenone for QS-6 and QS-10. Idebenone was metabolized and easily excreted and no accumulation was observed for the compound and its metabolites. No significant modification of the biohumoral indexes and vital signs and no adverse reactions were observed.