Muscle biopsy specimens from two patients with progressive myoclonus epilepsy (Lafora type) showed a prominent stippling pattern in muscle fibers with the nicotinamide adenine nucleotide dehydrogenase-tetrazolium reductase reaction, hematoxylin-eosin, and PAS stains. Ultrastructurally, stippling corresponds to small, membrane-bound packets of densely osmiophilic granules present in two sizes: one, 20 to 140 Angstroms; the other, identical to β-glycogen. Both are entirely removed after one-hour α-amylase digestion. In approximately 5% of the packets there are collections of 40 to 80 A fibrils strongly resembling cerebral Lafora bodies. Autophagic vacuoles appear in type 2A fibers. Material in the packets is probably a glycogen-like substance whose formation method is based on the general metabolic defect in progressive myoclonus epilepsy. The morphologic appearance of the tissue permits diagnosis using the benign technique of muscle biopsy.