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Background: Hippocampal volume reduction has been repeatedly demonstrated in schizophrenia. The relative contribution of genetic and environmental factors to this is unclear. Methods: To address this question, we compared volumetric measurements of the left and right hippocampus, obtained using stereological methods from brain MRI scans, from two groups of patients with schizophrenia as well as healthy controls ( n = 26). Patients ( n = 27) in the first group, had no family history of schizophrenia and had experienced severe pregnancy and birth complications (PBCs). The second group comprised of patients ( n = 21) without a history of severe PBCs from families multiply affected with schizophrenia. Results: Reduction of the left hippocampal volume was associated with the diagnosis of schizophrenia but was present only in patients with a history of severe PBCs; in this group the smaller the hippocampal volume, the earlier the onset of psychosis. Conclusions: Our findings suggest that environmental factors, in this case severe PBCs, make a significant contribution to hippocampal abnormalities in schizophrenia.