We made a comparative clinical, immunopathological and therapeutic evaluation in 17 patients with polymyositis (PM) and 12 patients with dermatomyositis (DM), followed up at our Neuromuscular Center. DM can be distinguished by its clinical appearance and pathological changes. Current evidence suggests that it results from vasculopathy. For studying these inflammatory myopathies we used multifactorial diagnostic criteria, evaluating the therapeutic response by means of a composite clinical and functional score in a longitudinal study. In muscle biopsy specimens we characterized with monoclonal antibodies T lymphocyte subpopulations (CD4, CD8), macrophages, IgG, IgM, C1q, C3, C4 complement fractions, MHC-I, MHC-II. In PM the cell-mediated immunity was more pronounced and in some cases both MHC-I and MHC-II molecules were found on the surface or within muscle fibers. Our patients were treated with steroids; in resistant cases azathioprine, cyclophosphamide, plasmapheresis, high-dose intravenous immunoglobulins (ivIgG) and total body irradiation were added to the therapeutic schedule.