The systemic juvenile idiopathic arthritis (SJIA) represents approximately 10% of all cases of JIA, there is no difference in onset rate between boys and girls. SJIA is characterized by pain of the joints, high, remitting fever, polymorphous skin rash and symptoms in other organs.

Hepatosplenomegaly, lymphadenopathy, serositis, features of acute inflammation are observed. Often there is pain of the joints. But myositis is observed rarely. The presented case is 22-years-old male suffering from the systemic JIA, with fevers and myositis symptoms (mainly intercostal muscles) dominating in the initial stage of the disease. Discussed was the clinical image, diagnostic issues and treatment.