A syndrome is described in which severe, clinically unexplained dyspnea is found at autopsy to be caused by multiple microscopic tumor emboli. Such a situation was found in 8 of 16 cases of multiple microscopic tumor emboli in the pulmonary arteries without significant lymphatic or parenchymal involvement of the lungs. The origins oftumor emboli included carcinomas of the prostate, breast, stomach, pancreas, and liver. In the 8 cases that presented with unexplained dyspnea, initial physical, roentgenographic, and electrocardiographic examinations were not diagnostic. Clinical or morphological evidence of car pulmonale was pre emboli did not invade the walls of pulmonary vessels but were frequently associated with thrombi. Recognition of this clinicopathologic entity becomes important as progress is made in cancer therapy.