Oligodendroglial microtubular tangles (OMT), a distinctive oligodendroglial change, was observed in seven of eight cases of olivopontocerebellar atrophy (OPCA). This change was a well-defined glassy cytoplasmic structure showing intense argyrophilia. OMT were distributed in the brain stem, cerebellum, and basal ganglia where severe neurodegenerative changes were consistently observed. In 45 control cases, no OMT were found regardless of the presence or absence of neurological disorders. The OMT were immunostained by anti-tubulin antibodies, but no other antibodies reacted with them. Each OMT consisted of a meshwork of randomly oriented fibrils studded with granular and fuzzy material. The fibrillary elements were between 20 and 30 nm in diameter. It is suggested that OMT are primarily composed of altered microtubules, and are related to the neurodegenerative process of OPCA.