The Jarisch-Herxheimer reaction (JHR) observed after antibiotic treatment of relapsing fever caused by Borrelia recurrentis is associated with the systemic appearance of cytokines. The decrease of cytokine production and block of JHR was attempted by administering pentoxifylline prior to antibiotic treatment. Fifteen patients with confirmed relapsing fever were infused intravenously with pentoxifylline 90 min before intramuscular injection of penicillin; 4 patients were not treated with pentoxifylline. All patients developed JHR to varying degrees. Treatment with pentoxifylline failed to prevent fever, increase in pulse, respiration, or blood pressure, or decrease in white blood cell count. No reduction of circulating levels of tumor necrosis factor, interleukin 6, or interleukin-8 was observed with pentoxifylline treatment. Pentoxifylline did not prevent clearance of the B. recurrentis spirochetes. Thus, pentoxifylline treatment of patients with relapsing fever fails to prevent or diminish JHR or the associated cytokine release observed after appropriate antibiotic treatment.