Objective. To study the effects of immunosuppres-sive therapy, in particular, corticosteroids, on morpho-logic signs of inflammation and expression of cytokines, adhesion molecules, and class I major histocompatibil-ity complex (MHC) antigen in muscle tissue from patients with polymyositis (PM) and dermatomyositis (DM) and to correlate the molecular changes with changes in muscle function. Methods. Seven patients with PM and 4 patients with DM underwent muscle biopsy before and after 3-6 months of therapy. Ten of the 11 patients were initially treated with prednisolone 30-60 mg/day. The phenotypes of infiltrating inflammatory cells and the expression of interleukin-1 (IL-1) and IL-1, adhesion molecules, and class I MHC antigen were studied by immunochemistry. Computerized image analysis was used for quantitation of staining. Muscle function was assessed with a muscle function index score. Results. Pronounced improvement of muscle function during the treatment period was noted in 8 of the 11 patients. The changes in muscle function coincided with an almost complete disappearance of inflam-matory cells, including CD3 T cells, in the patients with clinical improvement. These patients also exhibited decreased expression of IL-1, IL-1, intercellular adhesion molecule 1 (ICAM-1), vascular cell adhesion molecule 1 (VCAM-1), leukocyte function-associated antigen 1, and very late activation antigen 4. Of note, there was persistent expression of IL-1, ICAM-1, and VCAM-1 in capillaries and of class I MHC antigens on muscle fibers in several of the patients who, after corticosteroid treatment, still had muscle weakness despite the disappearance of inflammatory infiltrates. Conclusion. Changes in the muscle expression of key molecules in the inflammatory process, such as IL-1 and IL-1, ICAM-1 and class I MHC antigens, showed a consistent but not complete concordance with changes in and status of muscle function in patients with myositis who received the current standard treatment for the disease. These data indicate that it is possible to further evaluate various therapies for myo-sitis using molecular analysis of muscle biopsy specimens obtained on repeated occasions. In addition, the data demonstrate a dissociation between muscle function and degree of inflammatory infiltration in the affected muscles and suggest that the functional defects are more related to the expression of molecules such as IL-1 in muscle capillaries than to the mere presence of inflammatory cells in the affected muscles.