One hundred and forty-one seven-year-old Swedish children took part in an extensive neuropsychiatric study involving total population samples of children who had shown perceptual, motor and attentional deficits in pre-school, and blindly examined comparison children. The present paper reports on generalized hyperkinesis, i.e. hyperactivity in the child in all of three different assessment settings. About a third of children diagnosed according to strict criteria as suffering from 'Minimal Brain Dysfunction' showed generalized hyperkinesis. In children without perceptual-motor deficits, hyperkinesis was very much rarer. A total population frequency for generalized hyperkinesis in the range of 1-3% is reported as probable. Heredity for delayed maturation, non-optimal pre-, peri- and neonatal factors and adverse psychosocial factors were found to interact in the shaping of the disorder.