Abstract: Aim: The aim of this study was to assess the demography of polymyositis in Iranian children. Methods: 47 children classified by the ACR Criteria as having polymyositis were studied. 95 items were checked for every patient. They were followed regularly and the data were updated at each visit. Results: Thirty patients were female (64%). The acute presentation of the disease was seen in 10%. The manifestations at the onset of the disease were muscle weakness 60%, specific rash 43%, and arthralgia 43%. The clinical manifestation during the disease were: Constitutional symptoms 85%, proximal weakness 98%, cutaneous involvement 96% arthralgia 63%, arthritis 49%, cardiac manifestations 13%, pulmonary involvement 33.4%, and dysphagia 33.4%. Laboratory tests showed increased muscle enzymes in 100% of patient. Myogenic pattern in electromyography was seen in 98% and myositis pattern in muscle biopsy in 89% of patients. Conclusion: 1- The most common presentation was the insidious form. 2- the most common manifestation was the muscle weakness. 3- Osteoarticular involvement was not rare in Iranian children.