A 46-year-old man with nontropical sprue had anemia and hypoproteinemia for several years, until his condition was diagnosed and treated with dietary measures. Within a year after the diagnosis, progressive multifocal leukoencephalopathy developed, and the patient had a slightly fluctuating chronic downhill course until he died 10 years later. It is postulated that this patient's immune deficiency was related to his malabsorption syndrome and hypoglobulinemia, and the course became unusually protracted (longest reported course in the American literature) because of restoration of plasma protein levels. Autopsy showed the classic findings of progressive multifocal leukoencephalopathy, with much tissue loss of subcortical white matter and active perivascular inflammatory foci with numerous eosinophilic granulocytes. On electron microscopy, oligodendrocyte nuclei and cytoplasm were crowded with virions, but many myelin sheaths invested by severely infected oligodendrocytic processes were remarkably well preserved. This fact would argue against a direct cause-and-effect relationship between infection of oligodendrocytes and myelin breakdown in progressive multifocal leukoencephalopathy. The likelihood of an autoimmune mechanism at work in this disease is suggested, and the role of eosinophils and other cells in such process is considered.