Introduction A REVIEW of the available American and some foreign literature has failed to disclose a reference to a method of determining, from the roentgenogram itself, the degree of inspiration at which a radiograph of a chest is made. An x-ray examination of a co-operative patient assures a known phase of respiration if the proper radiographic technic is used. However, in younger children and unco-operative adults, the phase of respiration must be determined from the plates. It is the purpose of this paper to show that a knowledge of the phase of respiration is vital to the interpretation of a radiograph of the lung-fields, especially in children: also, it is our intent to describe a very simple method by which it may be determined by certain measurements on the roentgenogram itself. Roentgenograms made of healthy chests, when the lungs are in a state of incomplete inspiration, may show departures from the x-ray manifestations of normal lung-fields as they appear during complete inspiration which are practically indistinguishable from pathologic changes. Most of the x-ray evidence of pulmonary or cardiac disease, upon which diagnostic opinion is based, is composed of the following x-ray signs: 1. Degree of intensification of the trunk shadow; 2. Mediastinal and cardiac contour; 3. Outline of the thymus gland; 4. Size, shape, and density of the hilum shadows; 5. Degree of aëration of the lung parenchyma; 6. Presence or absence of parenchymal air displacement by infiltration or fibrosis; 7. Degree of visability, size, and shape of lymphoid elements; 8. Shape, position, and contour of the domes of the hemidiaphragms; 9. Position and spacing of the ribs; 10. Size, outline, and position of the trachea and main stem bronchi. We will present evidence which indicates that practically all of these signs change to a greater or less degree with respiration. If these signs do change as does the phase of respiration, it seems futile to render an opinion on the condition of the lung-fields of an unco-operative child or adult unless the degree of inspiration during which the x-ray plate was exposed is positively known. Evidence that respiration does alter these signs is furnished by Figure 1, which demonstrates the conspicuous diversity produced in the cases of three healthy cooperative children, by making plates at inspiration and at expiration in each case. These three children are clinically and radiographically (on other plates made at inspiration) free from any signs of pulmonary disease, and because of their cooperation we know the respiratory phase at which the plates are made. A, b, and c show complete expiration; d, e, and f are made at inspiration.