Background. Human infections with avian influenza A (H7N9) virus emerged in East China in March 2013. In contrast to seasonal influenza A viruses, H7N9 infections present a strong sex bias. Over the five epidemic waves in China, ~70% of all H7N9 cases were observed in men. We and others have independently shown that influenza A virus infections may significantly lower testosterone levels in male mice. In this study, we sought to analyze the role of testosterone in disease outcome in H7N9 infected men. Methods. We systematically analyzed a total of n=330 human samples obtained from male and female subjects of two age groups (18-49 years and ≥50 years) with laboratory-confirmed H7N9 influenza, seasonal influenza (H1N1, H3N2) or negative control cohorts including H7N9 virus-negative close contacts and influenza virus-negative poultry workers in dependency of sex. The level of testosterone, estradiol as well as cytokines and chemokines were measured and compared in all study participants. We used regression analysis to evaluate the association of sex hormones and cytokines in dependency of sex and disease outcome. Results. We analyzed sex hormones and innate immune responses in H7N9 cases (n=98), their H7N9 virus-negative close contacts (n=71), influenza virus-negative poultry workers (n=108) and mild seasonal (H1N1, H3N2) influenza cases (n=53). Samples were collected between 2014 and 2017. All control groups showed a similar median age within H7N9 cases except for the seasonal influenza group with a younger median age. In H7N9 infected men, testosterone levels were strongly reduced compared to male H7N9 virus-negative close contacts or males with seasonal influenza. Low testosterone levels in H7N9 infected men correlated with high inflammatory cytokine levels, e.g. IL-6, and lethal outcome in 18-49 year olds. No significant differences were detected in estradiol levels in H7N9 infected men. In H7N9 infected women (≥ 50 years), estradiol levels were elevated compared to H7N9 virus-negative close contacts without significantly affecting disease outcome. Conclusions. Here, we show that low testosterone levels pose a poor prognostic marker in H7N9 infected men. Thus, treatment of H7N9 infected patients should consider sex-specific mitigation strategies.