Objective: Inflammatory cytokines may play important roles in the pathogenesis of adult onset Still`s disease. The enhanced expression of IL-18 was reported in the bone marrow of a Japanese systemic onset juvenile rheumatoid arthritis patient but not in the other organs. To date, there are very few studies relating the bone marrow and AOSD. This study examined the bone marrow findings as well as TNF-α and IL-18 expression in the bone marrow of AOSD patients. Methods: A retrospective study was performed on 15 AOSD patients who had undergone a bone marrow examination at a university hospital. The clinical and laboratory findings, as well as the bone marrow findings, were analyzed. Immunohistochemistry of IL-18 and TNF-α in bone marrow was performed. Results: The bone marrow cellularity and myeloid/erythroid cell ratio showed no correlation with the clinical and laboratory findings. TNF-α was expressed at 0.8∼9.8% and IL-18 was expressed at 0.4∼9.8% of bone marrow cells. Cytokine expression was not associated with the clinical patterns of AOSD. The platelet count correlated with the bone marrow TNF-α expression but TNF-α did not correlate with IL-18 expression. Conclusion: TNF-α and IL-18 expression in bone marrow were observed in some AOSD patients but there was no correlation with the other clinical and laboratory findings except for the platelet count.