While several radiological findings point towards the basal ganglia as a possible anatomical site of the lesion in dystonia patients the biochemical basis of the disorder is still unknown. 5-Hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA) levels-the respective metabolites of serotonin and dopamine-were measured in lumbar cerebrospinal fluid (lCSF) of 15 patients with idiopathic adult-onset focal dystonia and in lCSF of 11 controls. 100 microliters lCSF were analyzed for 5-HIAA and HVA by reversed-phase HPLC with electrochemical detection. 5-HIAA levels were significantly reduced in dystonia patients (11.4 micrograms/ml) compared to controls (18.4 ng/ml) (p < 0.02). HVA levels in dystonia patients (30.3 ng/ml) were below control values (41.6 ng/ml) but this finding did not reach statistical significance. Decreased lCSF levels of 5-HIAA suggest an impaired serotonin metabolism in patients with idiopathic adult-onset dystonia. This observation may provide a biochemical basis for a more specific pharmacotherapy in dystonia patients.