Presently available data indicate that low-dose heparin prophylaxis will significantly diminish massive postoperative pulmonary emboli in patients more than 40 years of age subjected to major elective abdominothoracic surgery. The schedule is 5,000 USP units of heparin sodium subcutaneously, beginning two hours before surgery and continued every 12 hours (10,000 units/day) until the patient is discharged. Patients receiving this therapy should have a preoperative screening that includes a hematocrit reading, prothrombin time, partial thromboplastin time, and a platelet count. They should also not be receiving aspirin or other platelet antiaggregating agents for five days prior to surgery. The efficacy of this regimen is complemented by the fact that it is well tolerated by the patient, free of side effects, requires no laboratory monitoring, and produces minimal intraoperative or postoperative bleeding. This low-dose regimen has not proved effective in open prostatectomy or major orthopedic surgery. Data are not available concerning the drug's safety in spinal or epidural anesthesia, nor is it recommended for eye or brain surgery or in patients with an active thrombotic process. Other data are suggestive but still inconclusive that the regimen may reduce the incidence of postoperative acute myocardial infarction. In non-surgical patients hospitalized with acute myocardial infarction and receiving a low-dose heparin regimen, the findings reflect a significant decrease in deep venous thrombosis, though no observations are yet available concerning reductions in pulmonary emboli, mural thrombi, or systemic emboli.