Epidemiologic studies have reported that obstructive sleep apnea syndrome (OSAS) is a common disorder affecting about four percent of adult males and two percent of adult females. This difference in OSAS prevalence suggests that the female gender may reduce the risk of sleep breathing disorders in adults. We review several interrelated factors that may explain the differences in risk related to gender. These include differences in obesity and the distribution of adipose tissue, upper-airway anatomy, upper-airway muscle function, control of ventilation, the effect of sex hormones and leptin. The gender related protective effect decreases in females who are postmenopausal and not on hormone replacement therapy.