Tumor necrosis factor (TNF), a cytokine that has been implicated in the pathogenesis of many diseases including inflammatory disorders, has been found to be elevated in the muscles of patients with dermatomyositis (DM) and polymyositis (PM). A subset of patients with DM and PM do not respond readily to conventional therapy. Therefore, a controlled trial using infliximab, a chimeric IgG(1) kappa monoclonal antibody against TNF, may provide a therapeutic option and advance understanding in the pathogenesis of these diseases. This study is designed to determine the safety and efficacy of infliximab in patients with DM and PM. We plan to enroll a maximum of 28 patients randomized at 1:1 ratio into 2 groups, placebo vs. infliximab at 5 mg/kg body weight. In the first phase of the study the placebo and infliximab infusions will be given at week 0, 2, 6 and 14. Primary outcome assessment will be done at week 16. Those who respond according to the criteria set for improvement in MMT will be given the option to remain on the same infusion (i.e. placebo or infliximab at 5 mg/kg body weight) at weeks 22, 30 and 38. Placebo non-responders will be given infliximab at 5 mg/kg body weight in an open label fashion at weeks 16, 18, 22, 30 and 38. Patients who were initially on 5 mg/kg body weight of infliximab who failed to respond based on the criteria set will be placed on infliximab at 7.5 mg/kg body weight in an open label fashion on weeks 22, 30 and 38. Pharmacokinetic modeling will be done in both phases of the study. A muscle biopsy will be done before treatment and again at 16 weeks of treatment. Microarray gene expression profiling of the muscle biopsy specimens will be done before treatment and again at 16 weeks. Clinical, functional and serological evaluations will be performed in every visit to assess other responses to treatment. Patients will be followed for 30 weeks after study termination to evaluate the long-term effects of the drug.