A study of nonbacterial thrombotic endocarditis has been carried out in a series of 3,404 autopsies performed upon atomic bomb survivors in Hiroshima in the period 1953-1970. The prevalence of the lesion was 2.4 per cent, with a greater frequency among the elderly and among females, and with a significant relationship to malignant neoplasms. In contrast to other reported series, there was a greater prevalence among cancers of the colon, rectum, and female genitourinary tract. No relationship was noted between the presence of NBTE and exposure to ionizing radiation. Histologic findings in the heart-valve leaflets in close proximity to the verrucae, like experimental studies reported by others, suggest that in association with severe systemic disease there appears a process consisting of degenerative changes in valve collagen and ground substance, with subsequent denudation of endothelium, localized almost entirely to the apposing leaflet surfaces of the left-heart valves. The verrucae of nonbacterial thrombotic endocarditis are then formed upon this abnormal leaflet surface. While the relationship between systemic disease and the pathologic changes observed in cardiac valve tissue is unclear, and although it is not known whether a "hypercoagulable state" may accentuate the tendency for thrombi to form upon these abnormal valves, there is no doubt that this lesion represents a clinically important complication of severe systemic disease. It also seems likely that in some cases NBTE may complicate an illness which may otherwise be curable. Increasing awareness of this pathologic entity among clinicians, coupled with appropriate laboratory techniques, most likely echocardiography, will permit more frequent diagnosis in living patients.