The purpose of the present study is to investigate the hypothesis of association between the children's ordinal position in the sibship and the prevalence of schizophrenia. In order to examine the proposal above mentioned 404 schizophrenic pacients (201 males and 203 females) from five psychiatric hospitals in the Greater São Paulo were observed. The statistical method here employed, which permitted the analysis of the effects of birth order on the prevalence of schizophrenia, was that of Greenwood-Yule. It was found that there is an association between birth order and schizophrenia, and that this relation may be described in terms of an excess of schizophrenia among first-borns, both male and female.