It is well recognized that pulmonary function tests have aided in the early recognition of pulmonary dysfunction in patients considered to be normal on the basis of clinical and radiological examination, and in the differential diagnosis of patients with a known pulmonary disease, but in whom a specific diagnosis could not be made with certainty by other methods. The division of pulmonary function into the arbitrary categories of ventilation, distribution, and diffusion makes for easier discussion, but it should be noted that no single test is available at present which can be used to the exclusion of all others in the testing of these various categories. Different aspects of pulmonary function must be evaluated with different tests. Ventilation Ventilation may be defined as the process whereby air is moved into and out of the lungs. This aspect of pulmonary function, then, involves consideration of both the volumes of air moved, as well as of the factors inherent in moving the volumes of air, i.e., the mechanics of breathing. Ventilation and ventilatory impairment are usually evaluated with measurements of the conventional vital capacity, the timed vital capacity, and the maximal breathing capacity.' However, attention should first be directed to the concept of alveolar ventilation, since this is the volume of air which enters the alveoli with each inspiration and participates in active gas exchange. The amount of air moving into and out of the alveoli is obviously much more important than that moving in and out of the nose and throat. Alveolar ventilation may be evaluated clinically by observation of the uniformity and adequacy of the chest cage expansion, the extent of the diaphragmatic excursions, and by auscultation of the breath sounds.' Such ob~e~ations, however, are by no means quantitative insofar as volumes are concerned and may be misleading. Whenever there is any doubt as to the adequacy of alveolar ventilation, a quantitative determination should be done using the formula: Alveolar Ventilation=(Tidal vol. - Dead Space) x Frequency. The measurement of respiratory dead space is difficult, but in adults this can be considered to be approximately equal to the patient's estimated weight in pound^.^ Measurements of the vital capacity may be exceedingly difficult to evaluate, especially single measurements. A series of measurements, on