Body position during sleep influences the frequency of apneas and hypopneas in 50 to 60% of individuals with obstructive sleep apnea (OSA). In such cases, the apnea-hypopnea index (AHI) is increased in the supine posture and lower in the lateral posture. Positional sleep apnea is said to be present when there is a 50% reduction in the AHI during nonsupine sleep. Continuous positive airway pressure (CPAP) is a highly effective form of therapy for OSA. However, acceptance and compliance with CPAP are less than ideal. As an alternative to CPAP, patients with positional sleep apnea may be candidates for therapies that are designed to prevent the supine posture during sleep, ie, positional therapy. However, positional therapy is not likely to relieve symptoms if the AHI in the nonsupine position remains elevated. A more clinically appropriate definition would define positional sleep apnea when the rate of the AHI falls below the diagnostic threshold during sleep in the nonsupine posture. Thus, if positional therapy were totally effective in eliminating sleep in the supine position, the AHI would be normalized and positional therapy alone could be used for treating OSA in these patients.