There have been several studies showing the relationship between sex hormones and personality traits. Cytochrome P450 19 (CYP 19, aromatase) is the rate-limiting enzyme for the conversion of androgens into estrogens, and it has been reported that the C/T single nucleotide polymorphism in the 3′-untranslated region (3′UTR) of CYP19 gene (rs10046) affects dispositions of estradiol and testosterone. Therefore, it is hypothesized that this polymorphism affects personality traits. In the present study, the association of this polymorphism with personality traits was examined in 633 healthy Japanese subjects. Personality traits were assessed by the Temperament and Character Inventory (TCI), and the CYP19 genotype was detected by a PCR–RFLP method. In males, the group with the T allele predictive of elevated levels of estradiol and decreased levels of testosterone had lower scores of harm avoidance than that without the T allele (p = 0.015). In females, none of the seven TCI dimensions was different between the two genotype groups. The present study thus suggests that the C/T polymorphism in the 3′UTR of CYP19 gene affects personality trait of harm avoidance in healthy males.