The prevalence ratio per 100,000 population of each disease was multiple sclerosis (MS) 1.3, myasthenia gravis (MG) 6.7, and polymyositis (PM) 5.0 in Kumamoto city, Japan (population 0.56 million, and the survey date on June 30, 1982). The prevalence ratio of MS was similar to that of 25 years ago in Kumamoto city. The incidence of MS had been stable despite Westernization of life-style in Japan. The prevalence ratio of MG and PM had increased moderately, possibly due to prolongation of life with recent progress in therapy. The prevalence ratio of all types of myopathies in Kumamoto prefecture (population: 1.8 million) on July 31, 1983, was estimated as approximately 17.4 per 100,000, for progressive muscular dystrophy (PMD) 4.1 per 100,000 population, compared to 3.9 for MG, and 2.4 for PM. The relative frequency of PMD, MG and PM was 23.3, 22.0 and 13.9%, respectively. The data shows that the prevalence ratio of various myopathies has not changed over the last 20 years in selected cities of Japan but the relative frequency of the diseases have changed due to recognition of the disease and prolongation of long life due to developments in diagnosis and treatment.