Therapies for patients with polymyositis/dermatomyositis (PM/DM) who are resistant to steroids are limited, unproven and often toxic. In uncontrolled trials, both plasma exchange and leukapheresis have been reported to be of benefit, yet, despite widespread use, their efficacy is not established. We conducted a clinical trial to determine the value of apheresis in patients with idiopathic myositis. Thirty-nine steroid-resistant patients with biopsy-proven PM/DM were randomized to receive plasma exchange (n-13), leukapheresis (n=13), or sham apheresis (n=13). Twelve apheresis procedures were performed over a one month period. Muscle strength, functional capacity, and serum levels of muscle-associated enzymes were measured before and after the 12 apheresis sessions. Three patients in each group noted significant improvement in either muscle strength or function. However, despite a significant decrease in serum levels of muscle enzymes in the patients treated with plasma exchange compared with those receiving sham apheresis (p=0.0004), there were no significant differences in final muscle strength or functional capacity among the three treatment groups. We conclude that plasma exchange and leukapheresis as used in this study were no more effective than sham apheresis as treatment for patients with steroid- resistant polymyositis/dermatomyositis.