We designed a study to assess the specific contribution of cognitive dysfunction to multiple sclerosis patients' problems in daily living. Based on the results of a comprehensive neuropsychological test battery, we classified 100 MS patients as either cognitively intact (N = 52) or cognitively impaired (N = 48). In addition to a neurologic examination, MS patients completed questionnaires on mood and social functioning, underwent a comprehensive in-home occupational therapy evaluation, and were rated by a close relative or friend regarding specific personality characteristics. While there were no significant differences between the two groups on measures of physical disability and illness duration, patients in the cognitively impaired group were less likely to be working, engaged in fewer social and avocational activities, reported more sexual dysfunction, experienced greater difficulty in performing routine household tasks, and exhibited more psychopathology than cognitively intact patients. These findings suggest that cognitive dysfunction is a major factor in determining the quality of life of patients with MS.