The authors studied the ultrastructure of oligodendroglia and myelinated fibers and assessed quantitatively their density and proportion of pathologically changed myelinated fibers in the CA3 region of the hippocampus in 25 cases with schizophrenia and 25 control cases without mental pathology. Dystrophic changes of oligodendrocytes structure and an atrophy of part of myelinated fibers were found in schizophrenia. The severity of pathology was significantly associated with the age at disease onset. The neuropathological changes proved to be related with the reactivity of microglia that suggests a role of immune factors in the pathogenesis of disturbances observed in the study.