A prospective clinical and biochemical study on the effects of treatment with haloperidol has been performed in seven patients with Tourette syndrome. Pretreatment cerebrospinal fluid levels of homovanillic acid (CSF HVA) were significantly reduce in all patients, whereas 5-hydroxyindoleacetic acid was reduced in only two. With haloperidol treatment, symptoms decreased in all cases (21 to 88%) and clinical improvement was associated with an increased level of CSF HVA, often returning to the normal range. Optimal therapeutic response was found with serum levels of haloperidol between 1 and 4 ng/ml; however, disturbing side effects also occurred within this range. These results support the hypothesis that Tourette syndrome may result from a supersensitivity of dopaminergic receptors.