This prospective audit of incidence and outcome of the acute respiratory distress syndrome was conducted as part of the national audit of intensive care practice in Scotland. All patients with acute respiratory distress syndrome in 23 adult intensive care units were identified using the diagnostic criteria defined by the American-European Consensus Conference. Daily data collection was continued until death or intensive care unit discharge. Three hundred and sixty-nine patients were diagnosed with acute respiratory distress syndrome over the 8-month study period. The frequency of acute respiratory distress syndrome in the intensive care unit population was 8.1%; the incidence in the Scottish population was estimated at 16.0 cases.100,000(-1).year(-1). Intensive care unit mortality for acute respiratory distress syndrome was 53.1%, with a hospital mortality of 60.9%. In our national unselected population of critically ill patients, the overall outcome is comparable with published series (Acute Physiology and Chronic Health Evaluation II standardised mortality ratio = 0.99). However, mortality from acute respiratory distress syndrome in Scotland is substantially higher than in recent other series suggesting an improvement in outcome in this condition.