A controlled, prospective, double-blind, therapeutic trial of azathioprine was conducted in the initial therapy of polymyositis. Sixteen patients received 60 mg prednisone per day plus either azathioprine (2 mg/kg of body weight per day) or placebo for a period of 3 months. Creatine phosphokinase (CPK) levels fell to normal slightly sooner in the placebo group, but not significantly so. The azathioprine group did not become significantly stronger (P = 0.58) and did not manifest significantly greater improvement of histopathologic features of muscle (P = 0.80) than the placebo group. Initial CPK elevations were significantly related to the degree of muscle inflammation (P = 0.037), but this was not the case at 3 months (P greater than 0.05). Normalization of the CPK could not be equated with disease control. Type II fiber atrophy, attributed to steroid therapy, was more marked in women than in men (P less than 0.03).