The cytokine release syndrome (CRS) accompanying OKT3 therapy is a major cause of posttransplant morbidity. The pathogenesis of this syndrome has been attributed to the synthesis of tumor necrosis factor, interleukin 2 (IL-2), interleukin 6 (IL-6), and γ-inter-feron in response to T lymphocyte stimulation by 0KT3. The hemorrheologic agent pentoxifylline (PTX) inhibits the synthesis of TNFα in vitro in response to a variety of stimuli, including OKT3. We performed a randomized, double-blinded trial of PTX during 0KT3 induction in recipients of cadaveric renal allografts. Patients received either PTX 800 mg or placebo 2 hr before the initial dose of OKT3 and every 8 hr thereafter during the first 3 posttransplant days. Serum TNFa and IL-6 concentrations were measured pre-OKT3 and at 2 and 6 hr post-OKT3 on the first 3 posttransplant days.Despite the achievement of apparently adequate plasma levels of PTX and its active metabolites, no difference was observed in the incidence or severity of clinical manifestations of CRS. Serious manifestations of CRS—including acute pulmonary edema, encepha-lopathy, and aseptic meningitis—were not seen in either group. Serum TNFa and IL-6 concentrations were similar in PTX and control patients throughout the course of the study. Plasma levels of PTX and its active metabolites did not correlate with serum TNFa levels, serum IL-6 levels, or the incidence and severity of clinical manifestations of CRS.