We examined the incidence and mortality of adult respiratory distress syndrome (ARDS) in patients receiving emergency medical care at a large metropolitan medical center. The patients were classified into eight high-risk categories and monitored prospectively until discharge or death. Over a period of 12 months, 11,112 such patients entered the emergency room. Of 4926 who were admitted to the hospital acutely ill, 90 (2%) developed ARDS. Thirty-six percent of these survived. ARDS occurred in 25% of patients admitted with acquired immune deficiency syndrome and Pneumocystis carinii pneumonia, and mortality in these patients was 86%. Aspiration pneumonia was the primary cause of ARDS in 37% of the patients, who also had a similar mortality rate (85%). Mortality was also high in patients with multisystem organ failure or with ARDS and disseminated intravascular coagulation. These results indicate that there probably is a relatively low incidence of ARDS among hospital patients admitted with high-risk diagnoses.