Obstructive sleep apnea (OSA) is characterized by a high number of episodes of cessation of breathing during sleep, and is associated with oxygen desaturation, sleep fragmentation, and the symptoms of disruptive snoring and daytime sleepiness. It is estimated that 7 to 18 million people in the U.S. suffer from a mild form of OSA and that 1.8 to 4 million are afflicted with a more severe form. OSA has serious cardiovascular and neuro-psychological sequelae. The estimated cost of diagnosis and treatment ranges from $3 million, for the most severely affected to $40 billion including those with milder forms of OSA. In an initial sample of 91 families, odds of OSA is 1.3 times greater in subjects with one affected family member, and 2.3 times greater in subjects with three affected family members, than in controls, independent of age, sex, race, and obesity. This is suggestive of an inherited basis for OSA. Segregation analyses have indicated that the apopnea hypopnea index is distributed in families as a mixture of two distributions but does not segregate as a single Mendelian locus. Other ways of measuring clinical outcome are being investigated with a view to defining a simple mode of inheritance, if possible. Given the complex nature of OSA and the number of potentially informative covariates such as anatomic features (obesity and cranio-facial configuration) and pulmonary function, segregation analyses will be conducted with these covariates.