A physician's assessment of the probable outcome of an episode of respiratory failure should be based on a combination of survival data from large studies and specific knowledge about the individual case in question. Clearly, mortality rates in cases of ARF are influenced by a number of factors. In general, only a minority of patients with ARF complicating COPD require mechanical ventilation. In these cases, mortality often is related to the nature of the precipitating illness and the severity of the patient's underlying chronic respiratory disease. The long-term prognosis in patients with COPD who survive an episode of ARF is related primarily to the severity of the patient's underlying disease. Acute mortality is higher in patients with ARDS than in patients with ARF complicating COPD. Although a significant number of ARDS patients die of their underlying illness, mortality in others more commonly appears to be related to sepsis and multiple organ failure rather than end-stage respiratory disease. Pulmonary function in survivors of ARDS is quite variable, and may be related to the severity of the acute episode. ARF has a particularly poor prognosis when associated with certain underlying illnesses such as hematologic malignancy.