The idea that sex differences in the execution of different mental tasks mean a different cerebral organization met contradictory results. Sex differences in brain electrical activity were reported by various authors. The EEG coherences show functional and anatomical interrelation, so they may reveal differences in brain organization. This paper tries to demonstrate how sex differences are manifested in EEG coherences, in 42 right-handed control children, 18 girls and 24 boys, aged 7.6 to 13.3. Since the effect of age is high on EEG measurements, regression against age was calculated and eliminated. The Z-transformed coherence values were compared by Student's t-test. The results obtained showed that girls had higher right intrahemispheric coherence values than boys in all bands. These sex differences were more numerous in the theta band. The interhemispheric differences were slight and girls had higher coherences in all bands except for the alpha band, where boys had higher interhemispheric coherence than girls. These results show slight but consistent differences in the functional organization of the right hemisphere by sex and reveal the necessity to elaborate separated EEG norms for boys and girls.