The authors we investigated the relationship between plasma levels of haloperidol (HAL) and the number of CYP2D6*10 (*10) alleles in 66 Japanese inpatients with schizophrenia (male = 61, female = 5) on HAL. Plasma HAL level was determined by an enzyme immunoassay method. Daily dose of HAL was 1.5-36 (mean +/- SD = 12.3 +/- 7.6) mg or 0.02-0.49 (0.21 +/- 0.13) mg/kg body weight. Plasma HAL levels ranged from 1.4 to 47.4 (12.4 +/- 9.5) ng/mL. No significant difference in the plasma HAL levels was observed between the subjects with no, one, and two *10 alleles (one-way analysis of variance: 56.1 +/- 20.3, 61.0 +/- 20.3, and 63.3 +/- 20.3 ng/mL/mg/kg, respectively, F(2,63) = 0.65, p = 0.52). These results are not supportive of the previous report that plasma HAL levels can be predicted by the number of *10 alleles in Asian patients.