EEG coherence was studied in a group of n = 31 normal children (NG) and in a group of n = 25 mildly mentally retarded children (EG), 10-13 years old. This was done for a recording at rest, eyes closed, and one during which a visual matching task was presented. Coherence showed little structure across frequency, apart from a slow decline towards higher frequencies. For the EEG at rest, coherence was higher for the EG and a slight increase with age was found. The visual task changed the coherence pattern in a complex way, and differently for the two groups. Coherence decreased in the fronto-central region (about the same in both groups) and increased in occipito-parietal central combinations (more so for the NG). The delta band showed predominantly an increase for the NG.