To determine the incidence and 90-d mortality of acute respiratory failure (ARF), acute lung injury (ALI), and the acute respiratory distress syndrome (ARDS), we carried out an 8-wk prospective cohort study in Sweden, Denmark, and Iceland. All intensive care unit (ICU) admissions (n = 13,346) >/= 15 yr of age were assessed between October 6th and November 30th, 1997 in 132 of 150 ICUs with resources to treat patients with intubation and mechanical ventilation (I + MV) >/= 24 h. ARF was defined as I + MV >/= 24 h. ALI and ARDS were defined using criteria recommended by the American-European Consensus Conference on ARDS. Calculation to correct the incidence for unidentified subjects from nonparticipating ICUs was made. No correction for in- or out-migration from the study area was possible. The population in the three countries >/= 15 yr of age was 11.74 million. One thousand two hundred thirty-one ARF patients were included, 287 ALI and 221 ARDS patients were identified. The incidences were for ARF 77.6, for ALI 17.9, and for ARDS 13.5 patients per 100,000/yr. Ninety-day mortality was 41.0% for ARF, including ALI and ARDS patients, 42.2% for ALI not fulfilling ARDS criteria, and 41.2% for ARDS.