Objective To study the clinical and pathological manifestation of simple polymyositis. To investigate the expression of costimulatory molecules CD28/CTLA-4: B7 in peripheral blood lymphocytes and their roles in the pathogenesis of SPM.Methods The clinical situation, serum emzymes, electromyography(EMG) and muscular pathology of 141 patients with SPM were investigated. The expression of costimulatory molecules CD28, CTLA-4, B7-1, BB-1 and B7-2 in peripheral blood lymphocytes of six patients with simple polymyositis was measured with one-color flow cytometry(FCM). The control group were healthy volunteers.Results Muscle weakless, myalgia, elevation of creatine kinase and abnormal EMG of myogenic damage were very frequently to see in SPM. The muscle biopsy showed degeneration, necrosis and regeneration of muscle fibres, sporadic muscle fibre atrophy and endomysial inflammatory infiltration. The expression levels of costimulatory molecules CD28, CTLA-4, B7-1 and B7-2 in peripheral blood lymphocytes increased in SPM. Compared to control group, the mean fluorescence intensity of these molecules in SPM group showed by FCM increased remarkably (CD28, B7-2, P