Haloperidol and several other antipsychotic drugs are at least partially metabolized by the polymorphic cytochrome P450 2D6 (CYP2D6). The interindividual variation in metabolic capacity of CYP2D6 might be of importance when dosing. In this study, 26 outpatients with schizophrenia and depot haloperidol as monotherapy were genotyped. The authors found 1 patient with no functional alleles, 8 with one functional allele, 16 with two functional alleles, and 1 with three functional alleles. The daily dose of haloperidol ranged from 0.45 to 14.29 mg. Steady state plasma concentrations were measured at peak (range, 1.6-67 nmol/L) and at trough (range, 1.0-49 nmol/L). The Positive and Negative Syndrome scale for Schizophrenia and the Extrapyramidal Symptom Rating Scale were used to evaluate the clinical effect. The authors found a clear correlation between haloperidol plasma concentration and number of active CYP2D6 alleles. No correlation was found between plasma concentration of haloperidol or number of CYP2D6 alleles and treatment outcome or side effects. A model to predict plasma concentration from dose and number of active CYP2D6 alleles was formed from the obtained data by means of multiple linear regression.