All night EEG sleep recordings were compared in patients with dysthymia and major depressive disorder. Subjects were selected according to DSM-IV and underwent 2 weeks of treatment with placebo before the sleep evaluations. All patients with major depressive disorder were classified as nonmelancholic and belonged to the recurrent subtype, without full interepisode recovery. Patients with major depressive disorder have a shorter duration of total sleep time, a longer sleep latency and a lower sleep efficiency. However, similar sleep architecture and REM sleep characteristics were found in the two groups. These EEG sleep data seem to favor the existence of a biological overlap between the two forms of nonmelancholic unipolar depression. The validity of dysthymia is still unclear and therefore the nosological implications of our observations are discussed.