To determine: (1) the frequency of acute lung injury (ALI)/acute respiratory distress syndrome (ARDS); (2) the mortality associated with these syndromes and (3) the influence of risk factors, comorbidities and organ system dysfunction in the mortality of ALI patients. Prospective cohort study. Intensive care unit (ICU) of a general university hospital in Brazil. All patients that remained in the ICU for more than 24 h were evaluated regarding the presence/development of ALI/ARDS according to the 1994 American-European Consensus Conference. None. One thousand three hundred and one patients were studied and analyzed regarding mortality, risk factors, comorbidities and organ system dysfunction(s). The frequency of ALI was 3.8% (50), of which ARDS was 2.3% (30) and ALI/non-ARDS 1.5% (20) (p=0.15). The ICU mortality of patients with ALI was 44.0%; in ALI/non-ARDS and ARDS patients it was 40.0% and 46.7%, respectively (p=0.43). The hospital mortality of ALI patients was 48.0%; in ALI/non-ARDS and ARDS patients it was 50.0% and 46.7%, respectively (p=0.21). A multivariate analysis demonstrated that renal (ICU and hospital: p=0.002) and hematological dysfunction (ICU: p=0.008; hospital: p=0.02) were independently associated with ICU and hospital mortality in ALI patients. (1) The frequency of ALI was 3.8%, of which the frequency of ARDS was 2.3% and of ALI/non-ARDS 1.5%; (2) The ICU and hospital mortality of ALI patients was 44.0% and 48.0%, respectively; mortality rates of ARDS and ALI/non-ARDS did not differ significantly; (3) Renal and hematological dysfunction were associated with mortality in ALI patients.