Patients with polymyositis and dermatomyositis were earlier discouraged from active exercise due to a fear of increased muscle inflammation. There was also a lack of validated outcome measures to assess muscle impairment and activity limitation. The aim of this work was to evaluate safety and effects of different muscle training regimens and to develop and validate clinical outcome measures in polymyositis and dermatomyositis. A moderate-intensity resistive home exercise program was developed and performed five days a week for 12 weeks by patients with both established and recent onset, active disease. No signs of increased inflammation were detected and both patients with established and recent onset disease improved in muscle function and percieved health. A high-intensity resistance training were performed by patients with established disease resulting in improved muscle function and reduced disease activity. The Myositis Activities Profile was developed and validated assessing limitation in activities of daily living. The Functional Index 2 was developed assessing repetitive muscle function and validated for these patients.