Language and behavioral deviance in early childhood in preschizophrenia individuals suggests that the pathologic processes predisposing to schizophrenia are present from early in life. However, the etiologic antecedents of such impairments, and the degree to which they predict adult schizophrenia, have not been conclusively demonstrated. To address this, we examined language and behavioral predictors of adult psychiatric outcome in a population cohort (72 individuals with schizophrenia or schizoaffective disorder, 63 of their unaffected siblings, and 7,941 with no diagnosis) evaluated prospectively with behavioral examinations and a speech and language evaluation at 8 months, 4 years, and 7 years of age. Psychiatric outcome was ascertained via adult treatment contacts, and diagnoses were made by chart review according to DSM-IV criteria. Social maladjustment at age 7 was found to predict adult schizophrenia, and focal deviant behaviors (e.g., echolalia, meaningless laughter) at ages 4 and 7 were significantly associated with both schizophrenia and sibling status. Unintelligible speech at age 7 was a highly significant predictor of adult schizophrenia (odds ratio = 12.7), and poor expressive language ability predicted both schizophrenia and unaffected sibling outcome. Early behavioral and language dysfunction did not differentially characterize preschizophrenia subjects with a history of fetal hypoxia or an early age of first treatment contact. Given that unaffected siblings show similar signs of deviance, such problems may indicate genotypic susceptibility to the disorder, or shared environmental influences, or both.