This is a case report of juvenile idiopathic arthritis (JIA) in two pairs of brothers followed in the department of pediatric rheumatology, Universidade Federal da Bahia. Genetic involvement in JIA pathogenesis is clear and the risk of recurrence among siblings supports this contribution. An important landmark of this discovery involves the acknowledgment of major histocompatibility complex (MHC) polymorphism contribution to JIA development susceptibility. Despite many advances, the numerous available studies cannot explain several implicit mechanisms in JIA pathogenesis yet.

Riziform bodies are structures formed by fibrin and cells that can be found in the synovial fluid or attached to the synovium, and have this denomination due to its rice grain-like appearance. They have already been described in several diseases such as tuberculous arthritis, rheumatoid arthritis, and rarely in juvenile idiopathic arthritis (JIA). This is the case of a boy with a 4-month course of chronic monoarthritis of the left knee, with family history of sarcoidosis in which diagnostic investigation showed the presence of these riziform bodies in the synovial biopsy. Diagnostic investigation ruled out sarcoidosis, tuberculosis and malignancies, establishing the diagnosis of JIA. Our objective was to describe what we believe is the 9th case reported on the presence of riziform bodies in JIA, which are probably underdiagnosed, and should be considered mainly in cases of severe arthritis of difficult medical treatment.