OBJECTIVE: To investigate for differences in reproductive hormone levels in male relatives of women with polycystic ovary syndrome (PCOS). DESIGN: Cross-sectional study. SETTING: Academic medical center. PATIENT(S): Sixty-three fathers and 66 brothers of women with PCOS as well as two groups of control men of comparable age to fathers (older control, n = 30) and brothers (younger control, n = 58). INTERVENTION(S): A single early morning fasting blood sample was obtained for the measurement of reproductive hormone levels. MAIN OUTCOME MEASURE(S): Testosterone, LH, FSH, antimullerian hormone (AMH), inhibin B, estradiol (E2), and estrone (E1) levels were measured. RESULT(S): The AMH, LH, and FSH levels were significantly increased in male relatives compared with their respective control groups. The levels of E2, E1, T, and inhibin B did not differ between the groups. CONCLUSION(S): The AMH, LH, and FSH levels were increased in adult male relatives of women with PCOS, suggesting that they may have altered testicular function and changes in neuroendocrine regulation of gonadotropin secretion. These changes may reflect effects of PCOS susceptibility genes such as the recently mapped chromosome 11p14.1 locus in the region of the FSH B polypeptide gene.