Obstructive sleep apnea syndrome (OSAS) is the most common organic disorder of excessive daytime somnolence. In cross-sectional studies the minimum prevalence of OSAS among adult men is about one per cent. Prevalence is highest among men aged 40-65 years. The highest figures for this age group indicate that their prevalence of clinically significant OSAS may be 8.5% or higher. Habitual snoring is the most common symptom of OSAS (70-95%). The most significant risk factor for OSAS is obesity, especially upper body obesity. Other risk factors for snoring, and for OSAS, are male gender, age between 40 and 65 years, cigarette smoking, use of alcohol, and poor physical fitness. Upper airway obstruction with snoring or sleep apnea are commonly seen in children of all ages. Snoring is very common among infants and children with Pierre Robin syndrome and among infants with nasal obstruction. Snoring and obstructive sleep apnea are also very common in men with acromegaly. Many other syndromes or diseases exist in which the upper airway is narrowed. Prevalence of snoring and sleep apnea is increased in all such situations. It has been suggested that sleep apnea may be one mechanism contributing to sleep-related mortality. The prevalence of every night snoring seems to decrease after the age of 65. However, more than 25% of persons over 65 have more than five apneas per hour of sleep. It remains to be seen whether this finding has clinical significance. Partial upper airway obstruction, even without apneas, may influence pulmonary arterial pressure and may cause daytime sleepiness and some health consequences.(ABSTRACT TRUNCATED AT 250 WORDS)