Migrants to and from different risk areas for MS tend to retain much of the risk of their birthplace, but data from low-risk to high-risk regions are limited. For this purpose, Vietnamese (low risk) living in Paris (high risk) have been studied. The first stage of study was to identify all admissions to neurology services in Paris under ten diagnoses (MS and related disorders and ALS and related disorders) during 1965 though 1969. There were 4,067 such inpatients from 24 services of 14 hospitals; 16 other hospitals surveyed had no admissions in these categories. From the ratio of MS to ALS cases, it was thought that perhaps one half of the MS cases were missed. Accordingly, outpatient records from 13 services of 7 hospitals providing 90°/o of the MS hospital cases were included for the same 5 years. In the 13 services, total cases for MS and possible MS were then 3,402 and for ALS and possible ALS 454. From the ratio of definite MS to definite ALS cases (2,951/373) the prevalence of MS in Paris is estimated at 40/100,000 population, or 46 with possible MS. In the 220,000 clinic records of the 13 services, 355 were from Vietnamese: these included one case each of probable MS, bilateral RBN, ALS, PMMA, and possible PMMA. The ALS patient had been identified from the inpatient files. No Vietnamese with these diagnoses were found in the inpatient files of the other 11 services whose clinic files were not reviewed. While MS then seems rare among Vietnamese, even in Paris, definition of the population at risk with age at immigration is necessary for interpretation.