Our objective was to investigate the patterns of proliferation and differentiation of infiltrating cells in inflammatory myopathies. Immunohistochemical staining was performed on muscle biopsy specimens from 18 patients with inclusion body myositis, polymyositis and dermatomyositis using monoclonal and polyclonal antibodies. An abundance of cells were TNF-alpha+ (4-8%), ICAM-1+ (7-65%), IFN-gamma+ (3-6%), and Ki-67+ (4-8%). It was shown that 70% of the Ki-67+ cells were Ki-67+CD3+ cells. Very few mononuclear cells were IL-2R+. MHC-I expression was found on nearly all muscle fibres in all cases, while MHC-II expression was found on occasional muscle fibres in 1/3 of cases. Analysis of repeated biopsies from four IBM patients after prednisolone treatment showed no change in the proportions of TNF-alpha, ICAM-1, IFN-gamma or Ki-67 positive cells. In inflammatory myopathies there is an intense proliferation and differentiation of inflammatory cells in situ, indicating a local stimulation of the inflammatory process.