Radiofibrinogen leg scans were performed in 34 patients admitted to a respiratory intensive care unit (ICU) for treatment of acute respiratory failure. In 23 patients, the leg scans were performed during the first ICU week, and abnormal leg scans developed in three (13%). In 11 patients, leg scans were done beyond the first week; all were normal. Autopsy incidence of embolism was 20%. No patient died of embolism. The low frequency of fatal embolism had led us to discontinue the routine use of low-dose heparin prophylaxis in these patients in our respiratory ICU. In its place, we have substituted an ongoing investigation of noninvasive monitoring with both radioactive fibrinogen leg scanning and impedance plethysmography. From such investigations should emerge more definitive strategies for dealing with venous thromboembolism in this complex patient population.