A 7-day infusion of prostaglandin E1 (PGE1), an immunomodulator, was evaluated in a prospective, randomized, placebo-controlled, double-blinded trial in surgical patients with the adult respiratory distress syndrome (ARDS). The drug seemed to improve pulmonary function—only two PGE1 patients died with severe pulmonary failure compared with nine placebo patients (p = 0.01). Survival at 30 days after the end of the infusion— the predetermined end point of the study—was significantly better in the patients given PGE1 (p = 0.03), with 15 of 21 PGE1 patients (71%) alive at this time compared with seven of 20 placebo patients (35%). Improvement in overall survival in the PGE, patients did not reach statistical significance (p = 0.08). Overall survival in patients initially free of severe organ failure, however, was significantly better in the PGE, patients (p = 0.03). Of the six PGE1 patients free of severe organ failure at time of entry, all survived to leave the hospital; of the 10 placebo patients initially free of severe organ failure, four survived. The drug had no serious side effects and did not potentiate susceptibility to infection. PGE1 is a promising agent for the treatment of ARDS.