In the evaluation of pulmonary functions of patients, many physicians, even lung specialists and lung surgeons, state that lung function tests are unnecessary; they claim that the same information can be obtained by observing whether dyspnea is caused by ascent of a flight of stairs or by walking on the level. To prove or disprove this frequently stated opinion, the writers studied 200 consecutive hospital patients admitted for various cardiopulmonary diseases. The patients were asked whether ascent of one flight of stairs caused shortness of breath. This questioning was followed by routine ventilatory studies that consisted of forced expiratory volume (FEV), expressed as a percentage of the forced vital capacity (FVC), for the one-second interval (per cent FEV1), and expiratory peak flow rates, expressed in liters per minute and as per cent of predicted.