The introduction of low-dose heparin prophylaxis of thrombosis in deep leg veins represents a major advance in clinical medicine. It approaches an ideal form of prophylaxis in that it can be given easily to large numbers of patients at risk, requires no monitoring, is relatively safe, and, used widely, should save many lives. Conservative estimates are that 5,000 postoperative deaths and a comparable number of medical deaths can be avoided in the United States alone. Further inroads on this important problem will require additional developments, some of which are well under way.