In order to clarify whether or not marked changes in the social environment during the past 50 years in Japan may have altered the clinical phenotypes of multiple sclerosis (MS), we retrospectively analyzed 143 consecutive patients with clinically definite MS who developed the disease between 1950 and 1997. Fifty-two patients were classified as Asian type MS showing a selective involvement of the optic nerves and the spinal cord, while 91 patients were considered to have Western type MS which demonstrated the involvement of multiple sites in the central nervous system including the cerebrum, cerebellum and brainstem. The ratio of Asian type versus Western type MS was 1:0.5 in the patients born in the 1920s and 1:1.27, 1:1.64 and 1:1.7 in those born in the 1930s, 1940s and 1950s, respectively, and thereafter it increased to 1:4.67 in those born in the 1960s and 1:4 in those born after the 1970s. As a result, the proportion of Asian type MS significantly decreased in the patients born after 1960 as compared with those born from 1930 to 1959 (P=0.0121). In the Asian type MS, the age of onset was significantly higher in the patients who developed the disease from 1985 to 1997 (42.4+/-13.5 years) than in those who developed the disease from 1950 to 1984 (32.3+/-12.4 years) (P=0.0149), while in the Western type MS no such change in the age of onset was observed. These findings suggest that the frequency of Asian type MS has apparently decreased in younger Japanese born after 1960 when Japan's rapid economic growth had just started, and environmental factors are therefore considered to contribute to determine the clinical phenotypes of MS in Asians.