The term sleep-disordered breathing has been used synonymously with the term obstructive sleep apnea syndrome (OSAS). In a broader sense, however, the disorders of breathing during sleep exist along a spectrum of severity. The mildest form of sleep-related breathing disorder is intermittent snoring, which is primarily a nuisance without significant health sequelae. The most severe form of disordered breathing is the obesity-hypoventilation syndrome, which is associated with severe morbidity and very high mortality. In between these two extremes are disorders of gradually increasing impact on morbidity and mortality: persistent snoring, upper airway resistance syndrome, and OSAS.