Many neurological rating scales have been suggested to assess the impact of multiple sclerosis on patients, but none has been universally accepted. The Kurtzke Extended Disability Status Scale has been the most widely used despite its imperfections. It combines disability and impairment, has moderate inter-rater reliability, and its overall score is heavily weighted toward ambulation. The Scripps Neurological Rating Scale attempts to quantify impairment as measured by the traditional neurological examination. However, this and other impairment scales lack direct relevance to patients' functional health status. The Ambulation Index and some of the quantitative upper limb dysfunction assessment methods are sensitive and reproducible, but they only measure limited aspects of the wide range of disabilities encountered in multiple sclerosis. Current scales of disability and activities of daily living, such as the Incapacity Status Scale and the Functional Independent Measure, are not sensitive to the type of change which occurs in multiple sclerosis. The relationship between abnormalities on magnetic resonance images of the brain and disability has been difficult to ascertain. Although recently developed imaging acquisition methods may demonstrate abnormalities which are more closely correlated with disability, the demonstration of prevention, stabilization or recovery from disability using clinical scales will remain the final arbiter of success in clinical trials. We suggest guidelines for an improved disability scale.