Several lines of evidence suggest that the typically associated clinical features of Tourette's syndrome (TS), ie, tics, obsessive-compulsive behavior, inattention, and hyperactivity, commonly occur during childhood development. I hypothesize that TS is expressed in children as a clinical spectrum that includes a range of increasing functional impairment, indicating various degrees of abnormality in basal ganglia development. The mildest form, "developmental" TS, includes largely asymptomatic features and is estimated to occur in at least 3% of all children. Further along the spectrum are children with mild to moderate TS, who have school and behavioral problems; up to 25% of children requiring special education may be classified within this group. At the extreme end of the spectrum is the smallest group, patients with "full-blown" TS, characterized by more severe and often disabling symptoms. I propose that genetic influences are most important in determining the severity of TS along the clinical spectrum but that environmental factors may play a role. This hypothesis has implications for the understanding and treatment of childhood school and behavioral problems and the search for the TS genetic defect.