Research in hyperactivity has been primarily concerned with demon-strating casual relationships with cerebral dysfunction of the effects of drugs on improving learning performance. Studies which have attempted to focus on other factors influencing learning performance have demonstrated IQ and perceptual motor differences between hyperactive and normal children. More recently Koegel and Covert have shown that, among a group of autistic children, over-selectivity of components from stimulus complex may adversely influence learning performance on a simultaneous discrimination task. There have been no demonstrations of this phenomena with hyperactive children. The present study employed a simultaneous discrimination task composed of two stimulus complexes. Each complex in turn consisted of three geometric forms. Children were randomly selected, rated for activity level and then trained to respond reliably to specific complex. Once the discrimination to one of the complexes was established, the child was exposed to random pairs of components (one from the SD complex and one from the SΔ complex). Responses to either SD or SΔ were recorded. A significant relationship was found between over-selectivity for one or more SD components as a function of increasing activity level. Response errors were found to be related to selectivity but not to activity level. It would appear that performance deficits in learning a discrimination task may be, in part, related to over-selective focusing which results in failure to learn and hence, to integrate the entire stimulus complex.