In the two decades after obstructive sleep apnea syndrome (OSAS) was described, it was considered a disease primarily of males. As a result, for many years, epidemiologic studies of the general population examining the prevalence of OSAS included only males and investigators examined almost exclusively males in their pathophysiologic studies. It has been widely recognized that OSAS in women is not as rare as it was originally believed. Whereas early studies of clinic populations suggested that females made up about 10% or less of OSAS cases, later studies of the general population suggest that about a third of all cases are females. This suggests that there may be clinical under-recognition of OSAS in females. We explore the reasons for the male predominance of OSAS, and the clinical under-recognition in females by examining differences in clinical presentation and polysomnography findings between male and female patients.