The aim of this study was to investigate variability of morphological changes found in patients with sporadic inclusion body myositis, to assess the diagnostic value of muscle biopsy. The study included all 43 definite inclusion body myositis patients (86 biopsies) diagnosed at Sahlgrenska University Hospital, Gothenburg, Sweden, between 1984 and 2000. Invasion of mononuclear inflammatory cells in non-necrotic muscle fibres was found in 72 of 86 specimens, while all investigated biopsies showed up-regulation of major histocompatibility complex class I. Cytochrome c oxidase-negative muscle fibres were demonstrated in 84 of 86 biopsies. Rimmed vacuoles were present in all specimens from the vastus lateralis and tibialis muscles, and in 43 of 51 biopsies from the deltoid muscle. In cases with clinical suspicion of inclusion body myositis, where the muscle biopsy does not show inflammatory cell infiltration and rimmed vacuoles, inclusion body myositis should still be considered if there are cytochrome c oxidase-negative fibres and up-regulation of major histocompatibility complex class I. In such cases repeat muscle biopsy may be helpful.