1. Interhemispheric EEG differences were compared between 12 symptomatic depressed outpatients, 12 asymptomatic patients and 12 normal controls during two consecutive nights in the Sleep Study Unit. 2. EEG was quantified using digital period analysis (DPA), a time-domain analysis of successive polarity changes (zero-cross) and instances of zero slope (first derivative), yielding percent-time in each frequency band. 3. The degree of hemispheric asymmetry (L-R) was computed for delta, beta and theta percentages from REM, Stage 2 and Slow-Wave (SW) sleep. 4. Normals showed small asymmetries throughout sleep with largest differences in SW, with no consistent relationship between right and left activity and sleep stage. 5. Both depressed groups showed largest asymmetries in REM sleep, with significantly more beta, theta and delta in the right hemisphere consistently. None of the 24 depressed patients showed greater left hemisphere activity throughout sleep.