This report describes a sporadic late-onset myopathy in two unrelated adults which was marked by polyglucosan inclusions surrounded by abnormally structured mitochondria, the latter finding a localized, poss ibly reactive phenomenon. The polyglucosan material was characterized by a battery of histochemical and enzyme histochemical techniques; revealed common antigenicity with Lafora bodies, corpora amylacea and muscle fiber inclusions in types IV and VII glycogeneses; and contained ubiquitin, Additional lectin histochemical and associated digestion preparations disclosed the presence of alpha-glycosyl residues as apparently the sole carbohydrate component in polyglucosan bodies while the above mentioned common antigenicity with Lafora bodies and other inclusions suggests an additional, so far unidentified, protein component.