The incidence of fatal and nonfatal pulmonary embolism (PE) was evaluated in a series of 578 patients, to quantify the suspected higher risk in cancer patients of death from massive PE when compared with patients not having cancer. PE occurred in 13% and was fatal in 8% of noncancer patients. It occurred in 17% and was fatal in 14% (P less than .05) of cancer patients. Of these cases of PE, 75% occurred in patients with adenocarcinoma and 62% in those having tumors of the pancreas, breast, large bowel, prostate, lung, and ovary. One of every seven hospitalized cancer patients died not of cancer but of PE, and 60% of all patients who died of massive PE had localized cancer or limited metastatic disease which would have allowed for reasonably long survival in absence of lethal PE. Accordingly, we strongly suggest use of prophylactic anticoagulation in hospitalized cancer patients having otherwise good prospects for reasonably long survival.