The diagnosis of pneumothorax is established from the patients' history, physical examination and, where possible, by radiological investigations. Adult respiratory distress syndrome, pneumonia, and trauma are important predictors of pneumothorax, as are various practical procedures including mechanical ventilation, central line insertion, and surgical procedures in the thorax, head, and neck and abdomen. Examination should include an inspection of the ventilator observations and chest drainage systems as well as the patient's cardiovascular and respiratory systems.Radiological diagnosis is normally confined to plain frontal radiographs in the critically ill patient, although lateral images and computed tomography are also important. Situations are described where an abnormal lucency or an apparent lung edge may be confused with a pneumothorax. These may arise from outside the thoracic cavity or from lung abnormalities or abdominal viscera inside the chest. (Postgrad Med J 2000;76:399-404)