We review emerging evidence indicating that in utero exposure to infection is a risk factor for schizophrenia. It is hypothesized that a prenatal infection increases the liability to schizophrenia in adulthood by adversely affecting the maturation of critical brain structural and functional components implicated in the pathogenesis and pathophysiology of the disorder. Early evidence for a role of in utero infection includes investigations linking schizophrenia with birth during the winter and in urban regions, and ecologic studies demonstrating associations between influenza epidemics and births of pre-schizophrenic patients. The findings of the latter studies are, however, equivocal. To more rigorously address this question, our group has used increasingly sophisticated research designs that incorporate more refined measures of exposure and outcome, and continuous follow-up of treated cases. This work has already yielded several intriguing findings, including associations between schizophrenia and two in utero infections—rubella and respiratory infection. We also describe our ongoing birth cohort investigations that are expected to advance this work further, including studies that utilize maternal serum samples drawn during pregnancy of offspring who were later diagnosed with schizophrenia. MRDD Research Reviews 2002;8:51–57. © 2002 Wiley-Liss, Inc.