The aim of this study is to evaluate the association of the CYP3A4*18B genotype with the cyclosporine metabolism in healthy subjects. We employed PCR-RFLP assays for analysis of the CYP3A4*18B genotype. Each of 26 subjects, comprising 12 CYP3A4*1/*1, 12 CYP3A4*1/*18B and 2 CYP3A4*18B/*18B, was given a single oral dose of cyclosporine (4 mgkg(-1)). The plasma concentrations of cyclosporine were measured for up to 24 h post dose by high-performance liquid chromatography-electrospray mass spectrometry. We found that the mean Cmax (95% confidence intervals) of cyclosporine were 2237 (2905, 1859) (*1/*1), 2247 (2916, 1869) (*1/*18B), and 905 (1192, 506) ng ml(-1) (*18B/*18B)(p = 0.037) and the mean AUCO-4 were 5026 (6181, 4372) (*1/*1), 4434 (5481, 3841) (*1/*18B) and 2561 (3155, 1736) ng ml(-1) h (*18B/*18B) (p=0.021). The CL in the *18B/*18B group was significantly higher than in the *1/*1 group. However, Tmax exhibited no difference among the three genotypes. *18B/*18B group showed 50% reduction in concentration at 2 h post dose compared with *1/*18B (p = 0.062) or *1/*1 (p = 0.047), but no statistical significance was detected between*1/*1 and *1/*18B groups (p > 0.05). The data suggest that the CYP3A4*18B genotype affects cyclosporine pharmacokinetics probably resulting from a higher enzymatic activity of this mutation in healthy subjects.