Children and young adults of reproductive age have emerged as groups that are highly vulnerable to the current 2009 H1N1 pandemic. The sex of an individual is a fundamental factor that can influence exposure, susceptibility and immune responses to influenza. Worldwide, the incidence, disease burden, morbidity and mortality rates following exposure to the 2009 H1N1 influenza virus differ between males and females and are often age-dependent. Pregnancy and differences in the presentation of various risk factors contribute to the worse outcome of infection in women. Vaccination and antiviral treatment efficacy also vary in a sex-dependent manner. Finally, sex-specific genetic and hormonal differences may contribute to the severity of influenza and the clearance of viral infection. The contribution of sex and gender to influenza can only be determined by a greater consideration of these factors in clinical and epidemiological studies and increased research into the biological basis underlying these differences.