Obstructive sleep apnea syndrome (OSAS) was diagnosed in 157 subjects based on clinical symptoms, physical evaluation, cephalometric x-ray films, and polysomnography. These index cases identified 844 living first-degree relatives. Mailings were sent to 792 (94%). The mailing consisted of two identical questionnaires, one for the family member of the index case and one to be given to a friend (not a relative) of approximately the same age. In response, we received 531 (63%) questionnaires from relatives and 198 (25%) questionnaires from age-matched nonrelated friends, which were used as a control group. A more extensive investigation was performed on first-degree relatives of the index group living in the San Francisco Bay Area or vicinity. Two hundred seventy-nine relatives (100%) were identified. One hundred sixty-six subjects (59%) as well as 69 age-matched friends (ie, 41% of the 166 relatives and 25% of the potential total group) agreed to participate in further studies. These subjects had interviews, clinical investigations, and nonattended ambulatory monitoring. Cephalometric x-ray films could be obtained on only 22 of 166 participating relatives and 6 of 69 friends. Body mass index was not a differentiating measure between relatives and friends. Odds ratios (ORs) were calculated from the questionnaire data. The report of tiredness, fatigue, and sleepiness did not distinguish family members from friends. The OR, however, progressively increases when there is a positive history of near nightly loud snoring (OR=1.78; 95% confidence interval [CI]1.25-2.54) or a positive history of daytime sleepiness in conjunction with near nightly loud snoring (OR=3.11; 95% CI=1.94-4.99). The investigation in the Bay Area indicated that, when first-degree relatives were compared with friends, the complaint of daytime tiredness, sleepiness, or both with the presence of a high and narrow (ogival) hard palate sharply differentiated between friends and relatives (OR=10.9, 95%, CI=5.31-22.5). An Epworth Sleepiness Scale score of 9 or greater with the presence of another symptom associated with OSAS, and a respiratory disturbance index greater than 5(number of apneas and hypopneas per hour of sleep >5) gave an OR of 45.6(95% CI=18.8-11.0). Disproportionate craniofacial anatomy was common in familial groups with OSAS. Craniofacial familial features can be a strong indicator of risk for the development of OSAS.