To determine the epidemiology of dyspnea in terminal cancer patients, we examined data from the National Hospice Study, which followed up patients during their last six weeks of life. The incidence of dyspnea in these patients was 70.2 percent, with prevalence rates generally exceeding 50 percent at any of three measurements. In addition to lung or pleural involvement by the tumor, the presence of underlying lung disease or cardiac and low performance on the Karnofsky scale were significantly associated with dyspnea. Lung, colorectal, and breast carcinomas were the most common tumor sites in our dyspneic patients and accounted for almost 60 percent of cancer diagnoses in these patients. In 23.9 percent of dyspneic terminal cancer patients, neither lung or pleural involvement nor underlying lung or heart disease could be identified as risk factors.