Dogs with allografted lungs were studied to describe and to inter-relate serial changes in lung histology, plain chest roentgenography, and pulmonary angiography. Early rejection is associated with a perihilar alveolar infiltrate produced by edema and mononuclear cell infiltration. As rejection progresses, the chest roentgenograms show consolidation of the transplanted lung due to filling of the air spaces by organizing proteinaceous exudate and desquamated alveolar lining cells. Pulmonary angiography reveals slowing of flow through the transplant vasculature with failure to perfuse the peripheral vessels. Increased vascular resistance is chiefly due to direct involvement of the vessel wall by collars of mononuclear cells. With advanced rejection there is bronchial narrowing and decreasing aeration. Angiography then reveals a central extension of the zone of nonperfusion with narrowed and occluded peripheral branches of the pulmonary artery. Total rejection produces a poorly aerated, poorly perfused, retracted lung. Plain chest roentgenography is a good monitor of the progress of bronchial and alveolar rejection. Pulmonary angiography accurately reflects the stage of vascular involvement. Thus roentgenologic studies can be used to predict the histologic findings in untreated pulmonary allograft rejection.