To test the expanded definition of acute lung injury (ALI), we prospectively, over a period of 8 months, studied all adult ICU admissions who fitted the definition. Our study consisted of 83 patients with the adult respiratory distress syndrome (ARDS) and 60 with mild to moderate ALI. Sepsis and trauma were the most common diagnoses on admission. The overall mortality rate was 45 percent for ARDS and 38 percent in the other group. Mortality rose significantly with associated other organ failure, the incidence of which was as follows: hepatic, 39 percent; cardiac, 38 percent; hematologic, 22 percent; renal, 21 percent; neurologic, 5 percent. Sepsis syndrome eventually occurred in 73 percent and septic shock in 38 percent of all cases of ALI. We found the expanded definition a useful grading system and consider this definition of ARDS to be currently the best. There are, however, problems with the determination of lung compliance, the effect of inverse ratio ventilation, and the lack of consensus in defining other organ failure.