Clinical symptoms and signs and the frequency of abnormalities in the results of enzymatic, electromyographic, histological and serological tests were analysed in 50 patients, including 17 with polymyositis (PM) and 33 with dermatomyositis (DM), groups I and II according to Bohan's classification and followed-up for 25 years. Weakness of the proximal muscles of the extremities was present in both groups in all patients and in a high proportion of cases weakness involved also the distal muscles. Dysphagia was more frequent in DM patients (54.5%) than in PM (17.6%). In DM erythema and facial oedema as well as Gottron's sign were observed more frequently. In 11.8% of PM patients and 15.1% of DM patients deposition of calcium salts in subcutaneous tissue developed. Signs of vasculitis were found in 39.4% of DM cases and 17.6% of PM cases. In one-third of PM and in only 3 DM cases pitting oedema of the distal parts of the extremities was noted. Cardiovascular changes were disclosed in 82.3% of PM and 69.7% of DM patients. On the other hand, radiological signs of interstitial pulmonary fibrosis were noted more frequently in DM (36%) than in PM (23%). Increased serum activity of CPK, AspAT and ALAT was present with similar frequency in both groups from 71% to 81% of cases. EMG showed evidence of primary muscular changes in all PM patients and 69.6% of DM patients. Histological examination confirmed the diagnosis in the studied patients. Antinuclear antibodies were found relatively rarely, from 2% to 24% of PM/DM patients.(ABSTRACT TRUNCATED AT 250 WORDS)