Polymyositis (PM) and dermatomyositis (DM) are systemic inflammatory disorders affecting skeletal muscles and other organs, and are associated with high morbidity and mortality rates. In this study, we studied the prevalence, clinical features and its comparative outcome of PM/DM, comparing PM and DM. Twenty-three PM/DM patients (9 PM and 14 DM) were included in this study. The complication of interstitial pneumonia (IP) was found in 17 patients (74%). HRCT showed that non-specific interstitial pneumonia pattern was the most common in patterns of lung involvement. Twenty-one patients (91%) with PM/DM received high dose of prednisolone therapy. The percentage of patients who received methylprednisolone (mPSL) pulse and cyclosporin A was higher in DM patients than in PM patients. The percentage of patients who received mPSL pulse and cyclosporin A was higher in later (after Apr 2004) patients than in former (before Mar 2004) patients. Malignant diseases appeared in 3 patients with DM which consisted of breast cancer, epipharyngeal cancer and gastric cancer. We observed 2 deaths in DM patients during the course of therapy; one was due to IP, and the other due to miliary tuberculosis. This study showed that a poorer prognosis was observed in patients with DM when compared with those with PM, and immunosuppressive medications may be implicated at least partially in increased risk of infections and malignancies in PM/DM patients especially DM patients, indicating that patients with PM/DM may require careful monitoring during the clinical course.