From the epidemiological point of view, an observation was made on the features of Japanese B encephalitis, which occurred and prevailed in Okayama Prefecture, where the outbreak of this disease has been reported all these 8 years from 1946 till 1953. Almost every year, the number of patients in this prefecture exceeded the average one of the whole country. In addition, it seemed to indicate serious outbreak with the period of
about 10 years. Every outbreak has taken place between mid-August and early September. Both children and aged people were susceptible to this disease, the mortality being low in children and somewhat high in the aged patients. Male patients were more numerous than female patients. It is interesting to note that, when the local origin of the patients was investigated, it appeared that many children patients were of the southern parts of the prefecture and many aged patients of the northern parts, where the mortality caused by the disease is somewhat higher. Further investigation in this respect will be done in the future. The peak of this disease was usually seen to be reached in the south about two weeks earlier than in the north. Thus it may well be said that the features of the prevalence of this disease have an intimate relation with the climatic condition. The differential diagnosis of this disease was found to be difficult when accompanied with tuberculolus meningitis, ekiri, dysentery and some other disease with encephalitic or meningitic symptom.