In an investigation of 51 children with localized forebrain lesions (25 left hemispheric, 15 right hemispheric and 11 bilateral) a significant relationship between the extent of the cerebral lesion (quantitatively determined by means of computerized tomography scans) and the IQ was found, but only in children with lesions after age 5: there was a diminution of 3 (British Ability Scales and Porteus Mazes) to 4 (Wechsler test) points of IQ for a 1% increased brain lesion. There was no correlation between the extent of the lesion and IQ in children with early lesions. The IQ of the children with early lesions (up to year 4) was significantly lower than that of children with later lesions of the same extent. There was a highly significant correlation between behavioural problems (assessed by Conners Teacher Rating Scale) and diminution of IQ. There was a significant correlation between the extent of lesion and behavioural problems in lesions after year 5, but not in early lesions. While the correlation between intellectual-behavioural impairment and the extent of cerebral lesions can be explained by multiple localisation of functions, the lack of this correlation in early lesions and the more severe impairment by earlier lesions may be due to an interaction of brain development with social and lesional factors. Since each part of the brain may act as a stimulus for the development of other parts, even small differences in the lesions (and/or in the environment) may result in large behavioural differences (hypothesis of the developing brain as an amplifier for differences in the stimuli).