Most patients with the sleep apnea syndrome are male, and it has been suggested that there is a fundamental sex difference in breathing during sleep. To explore this we measured overnight ear oxygen saturation (SaO2), breathing patterns, and electroencephalographic sleep stage in 21 healthy nonobese women (10 premenopausal, 11 postmenopausal) and in 19 healthy nonobese men of similar age. Sleep duration averaged 6 h 51 min (range 343 to 468 min). The 21 older (greater than 45 yr) subjects had more episodes of apnea and hypopnea (p less than 0.01) and became more hypoxemic (p less than 0.02) than did the 20 younger subjects, but there was no sex difference, either in the younger or older subjects, in irregular breathing or nocturnal hypoxemia. Seven subjects 51 to 68 yr of age (4 men and 3 women) had more than 30 episodes of apnea and/or hypopnea and/or SaO2 decreases below 90%, but the clinical importance of this finding is unclear, 6 of the 7 being alive and 5 asymptomatic at an average of 47 months (range 5 to 67) later. Thus, irregular breathing and hypoxemia during sleep are common in both sexes older than 50 yr of age. Previous reports of a sex difference are probably explained by poor matching of the men and women, particularly with regard to body weight.