Multiple sclerosis (MS) and other chronic illnesses can drastically decrease quality of life (QOL), but there has been little systematic study of QOL in patients with chronic medical diseases. We analyzed QOL in 68 patients with MS, 164 patients with inflammatory bowel disease, and 75 patients with rheumatoid arthritis. The previously validated test instrument was a standardized interview consisting of 41 questions clustered in four subscales: functional and economic scale, social and recreational scale, affect and life in general scale, and medical problems scale. Patients were included in the study if they had a definite medical diagnosis and disease duration of 10 years or longer. In the patients with MS, Kurtzke's Expanded Disability Status Scale correlated strongly only with the medical problems score. Of Kurtzke's Functional System Scales, only the visual Functional System Scores was correlated with total QOL and subscale scores, suggesting that vision is strongly related to QOL. Duration of MS was unrelated to QOL scores. There were significant differences between patients with MS, inflammatory bowel disease, and rheumatoid arthritis on the subscale and total QOL scores. Results suggested that QOL was best in the inflammatory bowel disease group and worst in the MS group. Numerous statistically significant differences on individual questions were evident, suggesting that unique clinical profiles differentially characterize these diseases. Assessments of QOL are a meaningful addition to impairment scales, such as Kurtzke's Expanded Disability Status Scale. Furthermore, QOL scores may meaningfully measure the impact of a chronic medical disease, such as MS, compare the impacts of different diseases, and assess the effects of therapeutic intervention.