The etiology and treatment of a solitary bone cyst have remained undefined. Surgical treatments have not been encouraging, because a less invasive corticosteroid-injection treatment has afforded good results. However, there has been little scientific rationale supporting corticosteroid treatment. In recent reports, bone-resorbing factors, including matrix metalloproteinases, prostaglandins, interleukin-1, and oxygen free radicals, have been demonstrated in the cyst fluid. To better elucidate the pathophysiology of the solitary bone cyst, we examined the activities of nitric oxide and cytokines in the cyst fluid as well as in the cyst membrane. The levels of nitrate and nitrite were significantly higher in the cyst fluid than in serum. Immunostaining of cells in the stroma and lining cells of the cyst wall was strongly positive for inducible nitric synthase. The levels of interleukin-6 and interleukin-1beta in the cyst fluid were elevated, and cells in the cyst membrane were positive for tumor necrosis factor-alpha, interleukin-6, and interleukin-1beta. Cultured cells from the cyst membrane were induced in the production of nitrate and nitrite in response to cytokine treatment. These findings suggest that the solitary bone cyst was in a state favorable for the production of nitric oxide.