Skeletal muscle biopsies of 13 patients with polymyalgia rheumatica (PMR) were examined systematically with the electron microscope. Ultrastructural changes in the PMR specimens have been classified according to 14 criteria including nucleus, myofilaments, mitochondria, T-system, as well as glycogen deposition, lipid, lipofuscin, and myelin figures. The described muscle changes are of both regressive and progressive character and non-specific in themselves. The systematic recording of these criteria shows an extraordinaryly high incidence in PMR. This constellation of characteristics gives the ultrastructural picture of the skeletal musculature in PMR a definite profile. Similarities with the changes in other muscular diseases suggest that in PMR also neurogenous mechanisms may play a pathogenetic role.