Levels of thromboxane B2 (TxB2), the stable metabolite of thromboxane A2, are elevated in human and experimental septic shock. The thromboxane synthetase inhibitor dazoxiben has improved survival and decreased pulmonary hypertension in experimental endotoxemia. A randomized prospective study of 10 patients with the clinical diagnosis of sepsis and early adult respiratory distress syndrome (hypoxemia, radiologic evidence of the syndrome, and intrapulmonary shunt greater than 20%) was performed to test the efficacy of dazoxiben in ameliorating the effects of human sepsis. Five subjects received dazoxiben and five received placebo. Dazoxiben, 100 mg, or placebo was injected intravenously every 4 hours for a maximum of 72 hours. Plasma immunoreactive TxB2 (iTxB2) levels were determined by radioimmunoassay. Before dazoxiben, the plasma iTxB2 level was 752 +/- 261 pg/ml (n = 5) and was reduced within 1 hour to 333 +/- 137 pg/ml. The plasma levels of iTxB2 remained significantly decreased with subsequent doses of dazoxiben and it was 201 +/- 67 pg/ml (n = 4) 60 hours after dosing. In contrast, placebo had no significant effect on plasma iTxB2 levels (n = 5) throughout the entire period of observation. Dazoxiben did not induce any significant changes in pulmonary or systemic vascular resistance, intrapulmonary shunting, clotting studies, or extravascular lung water. One of the five subjects in the placebo group died and two of the five subjects in the dazoxiben group died. We conclude that dazoxiben was safe and effectively lowered plasma iTxB2 levels in patients with sepsis and incipient adult respiratory distress symptom, but did not significantly alter the hemodynamic and pulmonary sequelae of established sepsis.