A pair of identical twins (probability of monozygosity by genetic marking = 99.99%) with Gilles de la Tourette's syndrome (GTS) were studied clinically and neurochemically. The two boys were distinguished within the family by dramatic differences in their behavior and tics. Twin 1 had severe motor tics, loud vocalizations, coprolalia, and school behavior problems, whereas twin 2 was only mildly affected. Neither boy had ever been treated with medication for his tic disorder. Lumbar puncture was performed on each after a night of bed rest. In spite of the marked clinical differences between the two boys, the major indices of dopamine and serotonin turnover in the cerebrospinal fluid were equivalent. Homovanillic acid levels were 65 and 60 ng/ml, respectively, and 5-hydroxyindoleacetic acid levels were 36 and 39 ng/ml. Observation of these twins suggests that the phenotypic expression of GTS relates to more factors than inheritance. Neurochemical metabolites of dopamine and serotonin did not reflect the comparative clinical severity of GTS in these boys.