Invasion of nonnecrotic muscle fibers by cytotoxic T cells, accumulation of congophilic amyloid inclusions in muscle fibers, and fiber necrosis are consistent histologic findings in sporadic inclusion body myositis (IBM). To evaluate the relative significance of these alterations, we quantitatively analyzed the frequency of these abnormalities in 31 electron microscopy proven cases of IBM (20 untreated and 11 immunosuppressed). Nonnecrotic muscle fibers invaded by T cells were severalfold more frequent than fibers displaying the other pathologic alterations. Comparison of muscle samples from treated and untreated patients revealed no significant differences in the respective frequencies of the three species of abnormal fibers. Moreover, there was no correlation of the frequency of any abnormality either with disease duration or length of treatment. The much higher frequency of the invaded than Congo red-positive fibers points to the importance of an immune-mediated mechanism in the disease; but the basic cause of the disease remains undefined.