It is, therefore, important to further investigate the relationships between testosterone and erectile function, especially in a general population without the substantial biases inherent in patient samples. Previously in a sample of Korean men with lower urinary tract symptoms, free testosterone was correlated with erectile function, consistent with the later study, but total testosterone was not correlated with any of the five domains of the international Index of Erectile Function. In terms of other previous surveys, neither correlation between TT and ED risk nor with ED severity was demonstrated in studies of Brazil, Turkey and Italy, though low TT was associated with sexual dysfunction more often in the oldest subjects. And it was reported in the Olmsted County study, the age-related decline in sexual function was due to age-related declines in levels of BT Nilotinib rather than TT levels. It is only fairly recently that testosterone threshold for the relationship between TT and ED has been found in European Male Ageing Study. We conclude that the frustration to clarify the relationship between testosterone and ED in previous studies is probably due to the different provenances of studied population or the underpowered sample size. Moreover, to best of our knowledge, unhealthy lifestyles such as cigarette smoking, alcoholic drinking and physical activity, as well as the metabolic syndrome consisting of a myriad of abnormalities including central obesity, glucose intolerance, dyslipidemia, and hypertension have been associated with ED, but few studies considered these factors. It is, therefore, in order to further evaluate the relationship between testosterone and ED with the consideration of these confounders of ED, that we conducted this crosssectional study in a large series of Chinese men from general population. The present study in a large series of Chinese men from general population reveals that serum levels of FT and BT are decreased with age, whereas TT do not change much with age presumably because the SHBG increases as well. FT and BT are inversely related to worsening ED, whereas the positive association between TT and ED is most likely due to the increase in SHBG. The ED prevalence of our studied sample was 47.6%, closed to the prevalence of 49.4% in the primary care setting, 52% in Massachusetts Male Aging Study and 47% in BACH Survey. The IIEF-5 instrument was previously used in China, and it was reported that among Chinese men above 40 the prevalence of ED was 40.2%. Additionally, the BACH survey has demonstrated the contribution of modifiable lifestyle factors to the prevalence of ED. Consistent with the results from BACH, we have reported that heavy smokers had a significantly increased risk of ED than never smokers, which had some implications in the present study. Moreover, previous studies have reported that current smokers, alcoholic drinkers had a higher level of FT, and physical activity was positively associated with FT.