Testing the reliability and usefulness of disability scales in Parkinson's disease has been the object of a study carried out by 4 neurologists on 48 patients using 2 rating scales--Hoehn and Yahr staging and Columbia University Rating Scale--and 2 disability scales--Northwestern University Disability Scale and Extensive Disability Scale, a new scale conceived for this purpose, which is more accurate in examining in a different way the physical incapacity and handicap of parkinsonian patients in their daily living. The examiners worked in pairs and did not discuss their rating scores before the end of the study. As far as interobserver agreement is concerned, the results of the disability scales are better than those of the rating scales. The Northwestern Disability Scale and the new scale are similar in reliability and consistency; however, the Extensive Disability Scale has proved to be a better indicator of the functional status. We suggest a more extensive employment of disability scales in clinical studies.