To investigate human exposure to bisphenol A (BPA), a widely used endocrine disruptor, we measured serum BPA concentrations and analyzed the interrelation of BPA with sex-related hormones. BPA was detected in all human sera by a novel enzyme-linked immunosorbent assay. Serum BPA concentrations were significantly higher in normal men (1.49 ± 0.11 ng/ml; P < 0.01) and in women with polycystic ovary syndrome (1.04 ± 0.10 ng/ml; P < 0.05) compared with normal women (0.64 ± 0.10 ng/ml). There were significant positive correlations between serum BPA and total testosterone ( r = 0.595, P < 0.001) and free testosterone ( r = 0.609, P < 0.001) concentrations in all subjects and likewise between serum BPA and total testosterone ( r = 0.559, P < 0.01) and free testosterone ( r = 0.598, P < 0.001) concentrations in all female subjects, but not between serum BPA and other sex-related hormone concentrations in any group. These findings showed that there are gender differences in serum BPA concentrations, possibly due to differences in the androgen-related metabolism of BPA.