Metabolic saturation of voriconazole based on the trough plasma concentrations of voriconazole and its major metabolite N-oxide were evaluated according to CYP2C19 genotypes in 58 Japanese patients receiving voriconazole (median dose; 200 mg twice daily) for prophylaxis or treatment. Predose trough plasma concentrations of voriconazole and N-oxide were monitored on day 5 d or later after initiation of voriconazole treatment. Large interindividual variations in trough plasma concentrations of voriconazole and N-oxide were observed. Dose-normalized trough plasma concentrations of voriconazole were strongly correlated with its absolute trough concentrations, and the straight regression line between them intersected close to the origin of the coordinates. No significant correlation was observed between the trough plasma concentrations of voriconazole and N-oxide. The inverse value of the metabolic ratio of N-oxide to voriconazole was strongly correlated with the absolute trough voriconazole concentrations. No significant differences in the trough plasma concentrations of voriconazole and N-oxide or the metabolic ratio of N-oxide to voriconazole between the CYP2C19 genotypes were observed. Saturated metabolism of voriconazole N-oxidation rather than CYP2C19 genotypes contributed to the nonlinear pharmacokinetics. The metabolic process converting voriconazole to N-oxide was saturated at the clinical dose.