Aims: The general aims of the present study were to increase the understanding of the disease process in s-lBM by analysing the cytoskeletal structure in muscle fibres and their regeneration potential; and also to evaluate sensory function in patients with s-IBM to test the hypothesis of an associated neurogenic component or a concomitant neuropathy in s-IBM; and to evaluate therapeutic options by analysing the effect of IVIg treatment on the muscle inflammation; and to evaluate the effect of training on muscle function and muscle inflammation. Patients: Twenty-six patients with the diagnosis of s41BM participated in these studies. The diagnosis was based on clinical symptoms and signs as well as histopathological changes in muscle biopsy, using the diagnostic criteria for s-IBM according to Griggs et al. (1995). All patients had light microscopic muscle biopsy findings with inflammatory infiltrates including invasion of non-necrotic muscle fibres and rimmed vacuoles. The diagnosis was also verified by means of electron microscopy showing cytoplasmic tubulofilaments, 15-18 run in diameter. Methods: The cytoskeleton and regeneration activity in muscle from s-IBM patients were analysed by immunohistochemical methods, as was expression of T lymphocytes, macrophages, endothelial cells, cytokines, adhesion molecules and class I and II human leukocyte antigen (HLA). Repeated muscle biopsies were performed before and after IVIg treatment and before and after physical training. Safety and benefit of training was evaluated in a 12-week home training program. Quantitative sensibility tests and electrophysiology were used to analyse the sensory function in patients with s-IBM. Results: The expression of the intermediate filament vimentin and upregulation of desmin as well as expression of the satellite cell antigen CD56, indicates an ongoing active muscle fibre regeneration in s-IBM. This is ftirther supported by the presence of neonatal myosin. Sensibility dysfunction found in s-IBM patients indicates involvement of both ...