The study is cross-sectional and longitudinal in design. Index cases were identified through the Rhode Island Hospital Sleep Laboratory and controls through local industry. During home visits, questionnaire data on symptoms, medical history, and exposures were collected and the following measurements made: blood pressure, height, weight, and spirometry. Structural assessment of the upper airway was made by a brief physical examination, and facial structure was documented with a lateral photograph. Airflow, chest wall movement, oxygen saturation, and heart rate during sleep were recorded with an ambulatory monitoring device. Observations in the field were confirmed and extended with laboratory studies on a sample of families who demonstrated the greatest and the least concordance for sleep-related respiratory disturbances. These subjects had a more detailed assessment of upper airway structure with cephalometry and posterior rhinometry, assessment of ventilatory control with responses to chemical and resistive loading, and assessment of sleep staging with in-hospital polysomnography. Familial correlations with and without adjustment for specific risk factors were computed. These analyses allowed: a determination of the risk of development of sleep apnea due to familial factors; an improved understanding of the influences of genetic and acquired risk factors and their interactions on the development of sleep apnea: and characterization of a generally healthy population at increased risk for sleep apnea that were studied subsequently both longitudinally in natural history studies and with molecular genetic markers in pedigree studies.