Ninety-eight patients admitted to our respiratory intensive care unit during a one-year period were compared retrospectively with 99 well-matched patients admitted during a second one-year period. The use of prophylactic low-dose heparin in the second year was associated with a significant decrease in pulmonary emboli documented by ventilation-perfusion scan, pulmonary angiography, and autopsy. No specific bleeding complications could be directly attributed to the use of low-dose heparin. The frequency and severity of gastrointestinal hemorrhage as determined by hemoglobin fall and transfusion requirements were not significantly affected by the prophylactic use of low-dose heparin. Low-dose heparin appears to be effective and safe in respiratory intensive care unit patients in the prevention of pulmonary emboli.