When reviewed some 10 years ago, available prevalence studies of multiple sclerosis (MS) seemed to divide the world into three frequency zones for MS: high prevalence at 30 to 60 per 100,000 population; medium at 5 to 15; and low at less than 5 per 100,000. In the last decade the number of the available studies has more than tripled. Their reassessment, including judgments of comparability, still indicates a high-medium-low division for MS frequency world-wide. The high risk areas comprise northern Europe, northern United States, much of southern Canada, New Zealand, and probably southern Australia. Prevalence rates in these regions are mostly 30 to 80 per 100,000 population, centering at about $50. Medium frequency is defined as prevalence of 5 to 25, and is mostly 10 to 15. In Europe, the medium frequency zone bounds that of high frequency to the north, east, and south. The European Mediterranean basin is of medium prevalence with a sharp division from the high zone across France and Switzerland. It is likely that this division continues eastward across Austria, north of Hungary, and across the upper Ukraine to the Caspian Sea, but this is not definite. Medium risk areas of Europe thus include surveyed sites of Spain, Italy, Hungary, Jugoslavia, Bulgaria, and central Ukraine, together with southeastern France and southern Switzerland. Though Romania could be high, it is more likely to be of medium prevalence. Turkey measures low, but from incomplete data. From nationwide prevalence and mortality studies, the west coast of Norway and all Scandinavia above latitude 65° north are of medium frequency. Based on hospital data, northwestern USSR is high, and central and southern USSR medium, in MS risk. Other medium risk areas include southern United States, most of Australia, one ethnic group only in South Africa, and possibly Hawaii. Low risk areas are all surveyed sites of Asia, the Pacific islands, Africa, Latin America, Alaska, and Greenland.