Pulmonary embolism is a tragic and not infrequently fatal occurrence in hospitalixed patients. Approximately 2,700 persons die from pulmonary embolism in Great Britain and Ireland yearly. An association between pulmonary embolism and deep-vein thrombosis @VT) has been clearly documented’ If the incidence of pulmonary embolism is to be reduced it will be necessary either to prevent DVT or to detect it at a time when effective therapy may be introduced which will prevent pulmonary embolism. A number of prophylactic methods are in use for the prevention of DVT: “low-dose” heparin2 dextran,3 hydroxychloroquine sulfate,4 mechanical aida Each of these methods has been shown to reduce the incidence of DVT and, hopefully, future larger studies will show that one or a number of these methods is effective in reducing the incidence of pulmonary embolism. Alternatively, early diagnosis and treatment of DVT would lead to a reduction in the incidence of pulmonary embolism. Unfortunately the clinical diagnosis of DVT is notoriously unreliable. On the one hand it hae been shown that in a group of patients with clinical signs of DVT 28 per cent were normal by the sensitive lzsI fibrinogen test.6 On the other hand it has been reported that in 60 per cent of cases of pulmonary embolism clinical evidence of DVT is lacking.’ Consequently if DVT is to be diagnosed prior to pulmonary embolism it will be necessary to screen large numbers of clinically “normal” hospitalized patients at frequent intervals, possibly daily. A number of techniques are presently being tried as a screening test for DVT: the “‘1 fibrinogen test,’ thermography,’ electrical impedance,” ultrasound flow detection.” None of these tests to date is ideal as a mass screening test either because of inaccuracy in diagnosis or because of the complexity of the procedure or because the procedure carries a slight risk for the patient. Therefore, until a safe and effective prophylactic method for the prevention of pulmonary embolism is available, or until a simple, safe, and accurate mass screening test for the detection of DVT is developed, this common diagnostic and management question will continue to be asked in our hospitals: “Does this patient have DVT and should anticoagulant or antithrombolitic therapy be started?” To date, x-ray phlebography is the only method that will rapidly and accurately answer this question. A new bedside radioisotope test, the 1311 MAA clearance test, that my colleagues and 112 recently reported as a preliminary communication in the British Medical Journal would appear to have application in this area. This radioisotope test uses a commercially available radiopharmaceutical, ‘311-labeled macroaggregates of albumin (1311 MAA), and equipment that is portable and inexpensive. The results are available in 20 minutes. Labeled MAA has been in use as a lung-scanning agent for more than ten years. In 1969 Webber and associates’s re