Frequency of relapses in patients with polymyositis and dermatomyositis.

The frequency of clinical and biochemical relapses was determined in a group of 50 patients with polymyositis (PM), dermatomyositis (DM), or overlap syndromes who were followed for periods of up to 13 years. Relapses occurred in 30 of the 50 patients (60%) during the period of follow-up. The annual relapse rate was not significantly different in the three groups of patients. Subclinical relapses occurred in each group but were less frequent in the DM than in the PM and overlap groups. Relapses could occur at any time but were more frequent during periods of stable maintenance therapy. There was no correlation between relapses and initial disease severity, delay to diagnosis and commencement of treatment, or any class I or II histocompatibility locus antigen.