Two female patients of German origin, aged 38 and 21 years, with myoclonus epilepsy and cerebellar ataxia, but without dysmorphic signs and dementia, were found to excrete normal amounts of sialyl oligosaccharides in their urine. The younger patient showed cherry red spots in her ocular fundi. The older patient had a brother with an autopsy-proven neuronal storage disease compatible with sialidosis, and in her rectal biopsy lamellar inclusion bodies were detected. Enzyme assays in cultured fibroblasts of both patients revealed a profound but incomplete deficiency of oligosaccharide sialidase activity and normal beta-galactosidase activity. Adult sialidosis was diagnosed in both patients. In their fibroblasts, moderate elevations of bound sialic acid could also be measured. The small residual sialidase activity, which in the older patient had a normal KM value, is considered responsible for the late onset and slow clinical course of the disease. It is concluded that in adult sialidosis the extraneural storage process can be difficult to demonstrate in terms of metabolite accumulation or excretion during the course of intraneuronal storage.