Experimentally induced pulmonary thromboemboli in animals may undergo rapid, spontaneous resolution.1-5Little information is available, however, concerning the early fate of pulmonary thromboemboli in man. Sautter et al6described two patients with massive pulmonary thromboembolism in whom selective angiograms showed disappearance of the occlusive lesions within 25 and 128 days. Simon and Sasahara,7using the same technique, observed regression of filling defects in lobar arteries within eight weeks in one patient and complete clearing of multiple segmental obstructions within six weeks in another. Sabiston and Wagner,8using the method of radioisotope scanning in a patient with recurrent pulmonary thromboemboli, noted significant improvement in blood flow to the upper part of the right lung two months after the initial scan. In the study reported here, selective pulmonary arteriograms were performed serially to define more precisely the natural history of acute pulmonary thromboembolism in man. Material and Methods