In recent years, birth order has received particular emphasis in the study of structures and relationship in the families of schizophrenics. Several studies evidenced a higher risk of morbidity in firstborn children. The present research intends to verify the nature of this relationship in Brazil. One hundred fifty patients (69 men and 81 women), diagnosed as schizophrenics, were studied. The patients were interviewed and given a questionnaire on personal data. Non-sçhizophrenic, psychiatric patients were used as controls: 70 male alcoholics and 29 female, severe neurotics, all institutionalized. No significant difference was found as to birth order. Among males, 35,7% of the alcoholics are first borns, as against 24,6% of the schizophrenics; among females, the proportions were 17,2% and 8,6% for the neurotic and schizophrenic patients, respectively. There was, however, highly significant effect with respect to the sex of the preceding sibling. For schizophrenics, the sibling which precedes the patient in the birth order is, in most cases, of the opposite sex (68,2% for the males and 71,6% for the females). The converse was observed among the controle subjects; 55,7% of the male alcoholics are preceded by a brother and only 8,6% by a sister; of the female neurotic, 62,1% are preceded by a sister and 20,7% by a brother. According to these surprising results, it is not birth order which plaus a pathogenic role, but rather the relation to the preceding sibling of the opposite sex. Further study should confirm the specificity of this family relationship and analyse its pathogenic effect in the etiology of schizophrenia. 29 female, severe neurotics, all institutionalized. No significant difference was found as to birth order. Among males, 35,7% of the alcoholics are first borns, as against 24,6% of the schizophrenics; among females, the proportions were 17,2% and 8,6% for the neurotic and schizophrenic patients, respectively. There was, however, highly significant effect with respect to the sex of the preceding sibling. For schizophrenics, the sibling which precedes the patient in the birth order is, in most cases, of the opposite sex (68,2% for the males and 71,6% for the females). The converse was observed among the controle subjects; 55,7% of the male alcoholics are preceded by a brother and only 8,6% by a sister; of the female neurotic, 62,1% are preceded by a sister and 20,7% by a brother. According to these surprising results, it is not birth order which plaus a pathogenic role, but rather the relation to the preceding sibling of the opposite sex. Further study should confirm the specificity of this family relationship and analyse its pathogenic effect in the etiology of schizophrenia.