Previous work has indicated that low temporal coherence of ultradian sleep EEG rhythms is characteristic of depressed patients and women in particular. It may also be evident in depressed children and adolescents, although most published studies are limited in sample size. The present study evaluated temporal coherence of sleep EEG rhythms in 173 children and adolescents 8-17 years of age, including 97 who met criteria for major depressive disorder (MDD) and were symptomatic but unmedicated at the time of study and 76 healthy controls. Temporal coherence of all-night sleep EEG rhythms was evaluated on the second of two nights in the laboratory. Data were coded for diagnostic group, gender and age and subjected to MANOVAs. Temporal coherence was significantly lower in adolescents with MDD, compared to healthy controls. Findings were most robust for coherence between left and right beta and between delta and beta in both hemispheres. Both gender and age strongly influenced between-group differences, with the lowest temporal coherence among MDD girls, even in those under 13 years of age. In conclusion, early onset depression is associated with a reduction in synchronization of sleep EEG rhythms that shows a differential maturational course in boys and girls.