The High-Mobility Group Box 1 protein (HMGB1) is a known nuclear protein which may be released from the nucleus into the cytoplasm and the extracellular space. It is believed that the mobilized HMGB1 plays role in the autoimmune processes as an alarmin, stimulating the immune response. In addition, muscle regeneration and differentiation may also be altered in the inflammatory surroundings. Biopsy specimens derived from patients with idiopathic inflammatory myopathies (IIM) such as polymyositis or dermatomyositis were compared to muscle samples from patients undergoing surgical interventions for coxarthrosis. The biopsy and surgery specimens were used for Western blot analysis, for immunohistochemical detection of HMGB1 in histological preparations and for cell culturing to examine cell proliferation and differentiation. Our data show lower HMGB1 expression, impaired proliferation and slightly altered fusion capacity in the primary cell cultures started from IIM specimens than in cultures of coxarthrotic muscles. The ratio of regenerating muscle fibres with centralised nuclei (myotubes) is lower in the IIM samples than in the coxarthrotic ones but corticosteroid treatment shifts the ratio towards the coxarthrotic value. Our data suggest that the impaired regeneration capacity should also be considered to be behind the muscle weakness in IIM patients. The role of HMGB1 as a pathogenic signal requires further investigation.