A comparison of hyperactive boys with and without associated conduct disorder was made to determine the validity of a narrowly defined concept of pure hyperactivity. Although clinical differences between the two sub-groups supported the diagnostic distinction, few differences appeared with respect to various possibly causal background factors. A small group of boys with pure hyperactivity was separated, however, on the basis of subnormal IQ scores. In contrast to the hyperactive boys with normal IQ, these boys showed a more serious history of developmental disturbanes which appeared to be linked to organicity. It was concluded that pure hyeractivity is an elusive concept and probably defines a heterogeneous group of children whose primary problems are cognitive in nature.