Thirty asymptomatic men and 19 asymptomatic women were monitored during one night's sleep to determine the incidence of breathing abnormalities and oxygen desaturation in normal subjects. Twenty men accounted for 264 episodes of nocturnal oxygen desaturation or abnormal breathing. Women never experienced oxygen desaturation, and only three had a total of nine episodes of apnea. These sex differences were highly significant (P less than 0.003). In men, increasing age and obesity correlated positively with the incidence of nocturnal oxygen desaturation and abnormal breathing. Four asymptomatic men weighing more than 90 kg dropped their saturation to very low levels (68 to 72 per cent). Abnormal breathing and oxygen desaturation during sleep in subjects with chronic obstructive lung disease of the syndrome of hypersomnolence with periodic breathing may represent the superimposition of smoking or obesity on a normal tendency to snoring and oxygen desaturation in men.