An electron microscopic morphometric study of myelinated fibers and proportions of pathological myelinated fibers in postmortem prefrontal cortex, caudate nucleus and hippocampus was performed in 25 cases with schizophrenia and 25 normal controls. Pathology of myelinated fibers was similar in all brain structures in schizophrenia and included focal destruction of myelin sheaths and atrophy of axons. These changes were not correlated with age, postmortem delay or neuroleptic exposure but they were significantly positively correlated with the age at onset of disease. The proportion of myelinated fibers with axonal atrophy was the most prominent in the prefrontal cortex and significantly positively correlated with the proportion of these fibers in the caudate nucleus and hippocampus. The authors suggest that ultrastructural abnormalities of myelinated fibers in schizophrenia might contribute to functional disconnections of the prefrontal cortex with other brain structures.