Objective: Aromatase, the key enzyme involved in estrogen synthesis, is expressed in a variety of cells and tissues including human peripheral blood leukocytes (PBLs). The present study was designed to evaluate PBL aromatase gene expression in male and female subjects of different age groups. In addition, differences in gene expression during the follicular and luteal phase of the menstrual cycle in women, and before and after testosterone administration in men, were estimated.Design: Aromatase mRNA and protein were measured in PBLs obtained from young (n= 10) and postmenopausal women (n= 10), men (n= 15), and prepubertal children (n= 10). Aromatase mRNA and protein were also measured during the follicular and luteal phases of the menstrual cycle in women, and before and after the intramuscular administration of 250 mg testosterone enanthate in men.Methods and Results: Aromatase mRNA measured by real-time PCR in PBLs from women during the follicular phase was significantly higher than during the luteal phase of the menstrual cycle (P< 0.05). In men, PBL aromatase mRNA values increased significantly following testosterone administration (P< 0.05). PBL mRNA aromatase levels in women during the follicular phase and men after testosterone administration were significantly higher (one-way ANOVA;P< 0.05) than in any other group. Children, postmenopausal women, and women during the luteal phase showed the lowest aromatase mRNA expression. The results of the immunoblot analysis confirmed the data obtained by real-time PCR. A positive correlation between PBL aromatase mRNA values and plasma estradiol and estrone levels during the follicular phase of the menstrual cycle was observed in the group of adult women. No other correlations were found.Conclusions: The aromatase gene is differentially expressed in PBLs from women, men, and prepubertal children, indicating a sexual dimorphism in the enzyme expression and an important role of sex steroids in the modulation of aromatase gene expression.