Objective It has been suggested that cytochrome P450 2136 (CYP2D6) is involved in dopamine metabolism within the brain. The dopamine system is suggested to play a role in determining normal character. The purpose of this study was to examine whether character traits are dependent on cytochrome P450 2D6 activity. Methods We investigated the association between temperament and CYP2D6 gene polymorphism. The subjects were all Japanese and the polymorphism genotyped in the present study was CYP2D6*10. Character traits were assessed using the Temperament and Character Inventory. Results There was no overall or specific association between personality traits and the CYP2D6*10 allele and genotype frequencies. Conclusions The present results do not support the hypothesis that CYP2D6 activity affects temperament and character. (C) 2003 Lippincott Williams Wilkins.