Nine cases of multiple system atrophy and 1 case of autosomal dominant olivopontocerebellar atrophy (neuropathological diagnosis) were retrospectively examined for the presence of oligodendroglial inclusions. Clinical diagnosis in the first 9 cases had been: olivopontocerebellar atrophy (3 cases), atypical Parkinson's disease (2 cases), Shy-Drager syndrome (2 cases) and multiple system atrophy (1 case); one of the patients could not be included in any of the above mentioned groups. The oligodendroglial inclusions were argyrophilic and located in the cytoplasm around the nucleus. They were revealed by Bodian's method in all cases of multiple system atrophy. They were not found in the case of autosomal dominant olivopontocerebellar atrophy. They were labelled by anti-ubiquitin antibodies, and were negative with anti-tau antibodies. At electron microscopy, they consisted of rectilinear profiles coated with a fuzzy material (diameter: 20-33 nm); this aspect was compatible with microtubules. Oligodendroglial inclusions were prominent in regions selectively vulnerable in multiple system atrophy (tegmentum pontis, putamen, inferior olives, substantia nigra and cerebellar white matter), even in those areas where neuronal loss or fascicular atrophy were minimal or absent. They were also observed in regions considered to be spared in multiple system atrophy, such as the motor cortex and the corpus callosum. Argyrophilic oligodendroglial inclusions are an early and specific marker of multiple system atrophy. It is suggested that autosomal dominant olivopontocerebellar atrophy lacking oligodendroglial inclusions does not belong to multiple system atrophy.