Arylsulphatase A (ASA) activity was evaluated in 47 patients with a diagnosis of parkinsonism or essential tremor. Mean ASA activity was significantly reduced compared with both a healthy control group of 71 individuals (p < 0.01) and with a group of 44 neurological patients without movement disorders (p < 0.02). Using definite clinical criteria the patients were classified as typical or atypical with respect to Parkinson's disease (PD) or essential tremor (ET). A normal ASA level was found in all the cases showing typical clinical features (PD and ET), while ASA activity was significantly lowered (p < 0.01) in 55.6% of the atypical cases (Parkinsonian syndrome or symptomatic ET). Our data support the hypothesis of a non-casual association between low ASA level and the clinical features of parkinsonism or symptomatic ET.