We investigated the fine structural details of the presence of apparently newly formed oligodendrocytes within reactive astrocytes in white matter lesions obtained by biopsy from seven cases (3 multiple sclerosis (MS); 3 progressive multifocal leukoencephalopathy (PML); 1 with nonspecific reactive changes next to a sarcoid granuloma). Intact oligodendrocytes were found within astrocytic cytoplasm in two acute MS lesions and also in the reactive white matter lesion. The internalized cells appeared to lie within membrane-bound vacuoles. Formation of rudimentary junctions was observed between the internalized cells and host astrocytes. Sometimes more than one oligodendrocyte was seen in the same astrocyte. Our study suggests that this newly recognized interaction between astrocytes and oligodendrocytes is not restricted to acute MS lesions and probably represents emperipolesis rather than phagocytosis. This apparently nonspecific finding may be expected in any lesion with a proliferation of astrocytes and oligodendrocytes. The precise mechanism of this phenomenon or its functional significance is not entirely clear.