Borrelia burgdorferi, the causative agent of Lyme disease, is a potent inducer of interleukin-1 beta (IL-1 beta), a cytokine implicated in the pathogenesis of inflammatory arthritis. The balance between IL-1 and the IL-1 receptor antagonist (IL-1ra), a naturally occurring inhibitor of IL-1, might influence disease expression. To explore this possibility, we have done a retrospective study that compared the clinical course of Lyme arthritis in 83 patients with concentrations of IL-1 beta and IL-1ra in the patients' synovial fluid. Patients with high concentrations of IL-1ra and low concentrations of IL-1 beta had rapid resolution of attacks of arthritis, whereas patients with the reverse pattern of cytokine concentrations had long intervals to recovery. Thus, the balance between synovial fluid IL-1 beta and IL-1ra concentrations relates to the time to recovery from an episode of Lyme arthritis.