In a prospective study of sleep-disordered breathing among healthy elderly controls (N = 23), major depressives (N = 17), and demented patients with probable Alzheimer's disease (N = 21), sleep apnea (defined as an apnea index of 5 or more) was found in 42.9% of demented patients, 17.6% of depressives, and 4.3% of controls (chi 2 = 9.90, p less than .01). A significant association between sleep apnea and dementia of the Alzheimer type was found in women but not in men. Moreover, severity of dementia was significantly correlated with apnea index. Possible neuropathologic and clinical implications of these findings are discussed.