The clinical records of 304 patients with acute respiratory insufficiency were studied in retrospect. All of them had been treated in an Intensive Care Unit during its first year and a half of operation. One hundred and thirty-one patients suffered from chronic obstructive lung disease and acute episodes of chronic respiratory insufficiency. The remaining 173 had acute respiratory insufficiency due to various etiologies. The overall survival for patients with acute episodes of chronic respiratory insufficiency was 66 percent, significantly higher than the 53 percent survival of patients with acute conditions (p less than 0.02). Sixty percent of the chronic cases had to be treated with mechanical ventilation; the mortality rate was 45 percent. Mortality was 70 percent among individuals with acute conditions who received mechanical ventilation. Almost two thirds of deaths occurring in acute cases resulted from non-respiratory complications. Mortality due to respiratory causes was significantly higher in patients with chronic conditions (p less than 0.01). Twenty-three of the 171 patients given artificial ventilation (13 percent) presented secondary respiratory complications as a result. The incidence of pneumothorax was 5 percent and a statistically significant association between barotrauma and PEEP was found (p less than 0.05).