In a retrospective study of 30 patients with pneumonia treated in the intensive care unit, it was found that cultures from sputum and bronchial secretions did poorly correspond with blood cultures or serological tests. In only 15 of the patients a reliable etiological diagnosis was ever established. Mechanical ventilation was used in 22 patients, usually with a high oxygen need. At the start of this ventilation a significant blood pressure fall and a further pulmonary deterioration was observed. In fatal cases this pulmonary dysfunction was progressive. The overall mortality was 47%. When an FI₀₂ above 0.6 was needed in the ventilator the mortality was 13/14 (93%).