The characterisitcs and performance of the Vitalograph dry spirometer were studied, and a direct comparison was made with the Stead-Wells spirometer, as recommended by the Committees on Environmental Health and Respiratory Physiology of the American College of Chest Physicians. The Vitalograph calibrates well against the Stead-Wells spirometer for measurements of vital capacity and forced expiratory volume in one second in normal subjects and in patients with various pulmonary conditions. Both spirometers failed to satisfy the committees' recommendations for resistance and inertia, the Stead-Wells showing the better values. Nevertheless, it is suggested that the Vitalograph is very suitable for the routine assessment of ventilatory function in the laboratory or in population surveys. Its light weight, portability, and robustness give it an added advantage.