Of 16 patients treated with intrapulmonary heparin at doses between 10,000 and 20,000 U/week for 1592 patient days, or 4.3 years, only one rethrombosed. This patient has a congenital antithrombin III deficiency. However, the use of intrapulmonary heparin, even in this particular patient, has remarkably decreased her thrombotic events as manifested by studying her history of deep vein thrombosis and pulmonary embolism prior to starting intrapulmonary heparin. This represents a failure rate of 4.2% in the total of 1592 patient days of therapy, or a rethrombosis rate of 1.4% per year. This recurrence rate is far superior to that reported for warfarin-type therapy or for platelet suppressive therapy. From this limited experience, it appears that heparin is an extremely safe and highly effective mode of outpatient prophylaxis for deep vein thrombosis and thromboembolic disease. The ultimate aim of this study is to determine the possibility of calcium heparin being placed into a hand-held aerosol nebulizer that a patient can use at home on a weekly basis. This would provide a highly convenient, safe, and apparently very efficacious mode of therapy for the long-term outpatient prophylaxis of deep vein thrombosis and thromboembolic disease.