In order to evaluate the impact of aging on EEG sleep patterns we investigated the polysomnograms of 74 patients with major depression and 51 healthy volunteers aged 18-65 years. In most of the EEG sleep parameters, age-related changes were obvious in both the depressives and the normals. In the patients, some of these alterations occurred earlier and were more pronounced. The amount of slow-wave sleep decreased with age, but no differences were found between the depressives and the healthy volunteers at any particular age. Rapid-eye-movement (REM) latency was clearly affected by age, but there were no significant differences between patients and controls until the middle of the fourth decade of life. On the other hand, REM density measures did not vary with age and were increased in the depressives. Therefore, REM density appears to be a more likely candidate for a biologic marker for major depression than is REM latency.