This longitudinal study has extended an earlier cross-sectional one in order to investigate the development of background activity and the topographical distribution of different EEG parameters during childhood and adolescence. Ninety-six healthy children (47 boys and 49 girls) were followed for up to 7 years with serial EEGs. These were analysed by spectral analysis. Power spectra were calculated for conventional frequency bands. Three topics were followed: (i) differentiation between inter-individual and age-related changes, (ii) evaluation of sex differences and (iii) evaluation of developmental velocity of different EEG parameters. Between 4 and 17 years the content of theta waves decreases and the content of alpha waves increases. In girls more than in boys, slow alpha waves (7.5-9.5 c/sec) in occipital leads first increase, as theta decreases. Later they decrease themselves while fast alpha waves (9.5-12.5 c/sec) increase. The velocity of occipital changes is almost twice that centrally, and is greater in early childhood than after 10 years of age. Girls under 6 years of age show significantly more theta and less alpha waves compared with boys. However, the developmental velocity (change per year) is higher in girls than in boys and continues for a longer time; thus girls catch up with age-related changes of the boys by a later developmental spurt. Individual characteristics of the subjects determine the EEG twice as much as the age factor; this explains the wide range of results in previous studies. No trend was found to suggest earlier maturation in one hemisphere while in 5% of the subjects the amount of activity for one parameter in one hemisphere exceeds 1.5 times the amount of activity in the other. The individual characteristics of the EEG spectra show a remarkable stability over the years.