The diagnosis of juvenile idiopathic arthritis (JIA) is an exclusion one due to heterogeneous clinical presentation and lack of specific laboratory tests. The authors investigated bone marrow of 25 untreated children with JIA at the onset of the disease. Bone marrow smears were evaluated for cell populations as well as myelodysplastic features and compared to two control groups. The characteristic of bone marrow in JIA was myeloid hyperplasia and elevated plasmocyte count. There was no difference between JIA patients and control groups in terms of myelodysplastic features. These findings can be helpful in explaining hematological alterations in JIA.