Based on the prevalence of multiple sclerosis (MS) in 10 Japanese cities, as studied by the MS Research Committee of Japan since 1972, the relationship between the prevalence ratio and latitude was tested using Armitage''s tests for linear trends in proportions and frequencies. Crude prevalence ratios were 0.8–4.0 per 100,000 population. With respect to both crude and age- and sex-adjusted prevalence ratios, a statistically significant north-to-south gradient for MS was found in Japan. This result may be explained by the existence of environmental factor(s), although it does not exclude the participation of immunologic processes as an additional causative or endogenous modifying facto