The physiological significance of cytochrome c oxidase (CCO) deficiency in rimmed vacuole myopathy (RVM) is not fully understood. Frequencies of CCO negative muscle fibers (CCO(-)F) were compared with two cases of in-clusion body myositis (IBM) and another two cases with RVM without inflammatory changes (i.e. oculopharyngeal distal myopathy (OPDM) and distal myopathy with rimmed vacuole formation (DMRV)). The frequencies of CCO(-)F were 6.9% in the case of definite IBM with 10 years of disease duration, 0.3% in possible IBM of 1 year duration, 1.3% in OPDM of 11 years duration, and 0.1% in DMRV of 2 years duration. The frequencies of CCO(-)F were generally higher in RVM than in controls. However, the incidence of CCO(-)F increased with time in patients with IBM compared with patients with RVM without inflammation. The present findings may provide important information on the myopathological distinction of IBM and RVM without inflammation.