An excess of schizophrenic births in winter or early spring is a well replicated finding in studies on Western populations. However, there are few studies on Asian populations, and their results are not consistent. We examined the seasonality of schizophrenic births in Japanese subjects (n = 1024) born between 1955 and 1960. Our results showed a significant inverse relationship between temperature and number of schizophrenic births, which is in line with the findings in Western populations. In our data, observed reduction of schizophrenic births in summer was more marked than excess births in winter. We suggest the possibility that birth in summer may have some advantage in brain growth early in life, which results in reducing the risk of developing schizophrenia.