Seventy-five patients with malignant neoplastic disease and nonbacterial thrombotic endocarditis (NBTE) were studied. The over-all frequency of NBTE was double that observed in other reported autopsy series not limited to patients with cancer. The incidence in autopsy patients with bronchiolar and adenocarcinoma of the lung was twice that of patients with pancreatic and prostatic adenocarcinoma and seven times that of patients with breast cancer. The development of NBTE could not be ascribed to duration of illness or nutritional state. Since 14 patients died of cerebral infarcts and five had major myocardial infarcts resulting from thromboemboli, these and other complications of NBTE should be anticipated particularly in patients with those cancers most often associated with vegetative endocarditis.