The objective was to investigate whether a 12-week resistive home exercise program in addition to conventional medical treatment could be safely performed regarding muscle inflammation, muscle function, and quality of life in patients with active polymyositis (PM) or dermatomyositis (DM). Eleven patients diagnosed with active PM or DM were included. Muscle biopsies and Magnetic Resonance Imaging (MRI) of the thighs were performed. Quality of life, function, and subjective global disease impact (SGDI) were assessed and creatine phosphokinase levels (CPK) were analysed. The patients exercised with the exercise program for 15 minutes and took a 15-minute walk five days a week for 12 weeks. After the exercise period there was no sign of increased muscle inflammation. The group showed significantly improved function and quality of life compared to the start of study. It seems that this exercise program safely can be employed in patients with active PM or DM, and we suggest that physical exercise should be included in the rehabilitation of these patients.