A countrywide survey of multiple sclerosis disclosed 282 patients living in Israel on Jan. 1, 1960. In 266 of these patients, clinical data were adequate for analysis and age at onset was known. In the present study, the patients were studied to determine whether clinical course and prognosis was influenced by the age at onset. Of the total group, slightly over half had an age-onset between 20 and 39 years, almost one-third after 40 and 15% before 20. Thus, the curve of age at onset of multiple sclerosis was skewed to the left. The proportion of females was greater among patients with early onset, while males predominated among those with onset after 40. The proportion of patients with a progressive course as opposed to a remitting course increased with age at onset. However, the rate of exacerbations per person-year of illness was approximately the same regardless of whether the disease began at a young age or later in life. Classification of patients by major neurological deficit showed a higher percentage of disseminated types in patients with early onset. The spinal types in patients with early onset. The spinal type tended to be commoner in patients with later age at onset. Visual loss and diplopia were reported more often as initial manifestations of multiple sclerosis by patients with early onset. After the same duration of illness, disability tended to be greater for patients with late onset and also for patients with a progressive course. It was concluded that there are appreciable clinical differences between patients with early onset as compared with those reporting late onset of the disease. These differences should be taken into account in evaluating the likelihood of multiple sclerosis at different ages of onset.