While genetic factors play a role in the etiology of schizophrenia, environmental factors contribute to the etiology of the disorder as well. If a woman is exposed during the second trimester of gestation to stressors such as the death of a loved one, influenza infection, or natural and man-made disasters, her baby has an increased risk of developing schizophrenia later in life. A common mechanism may mediate the effects of these diverse risk factors: a stress response during a restricted period of gestation may affect fetal brain development and, given the appropriate genetic predisposition, result in schizophrenia in adult life. Findings in animals exposed to prenatal stress strengthen the hypothesis that exposure to high glucocorticoid levels and/or other components of the stress response increases the risk of schizophrenia. These clinical and preclinical studies could be used to develop hypotheses that could then be tested in patients with schizophrenia.