A cohort of 886 cases of multiple sclerosis (MS) who had onset between Jan 1, 1960, and Dec 31, 1969, and who were resident in 1970 in either a low-prevalence area (Los Angeles County, California, 560 cases) or in a high-prevalence area (King and Pierce Counties, Washington, 326 cases) were followed up for changes in disability status through Dec 31, 1979. Cases had to meet the modified Schumacher criteria for definite/probable MS and to have had a diagnosis of MS or equivalent by a physician. Onset year of disease was determined by onset of the earliest reported symptom. Disability status was determined at intake by an interviewer-administered questionnaire and in subsequent years by an annual mailed questionnaire (self-reported disability status). Progression to a nonambulatory status or death was significantly greater among (1) patients with an older age at onset, (2) patients with a rapid early course, (3) males, and (4) residents of Los Angeles County.