For a long time, the studies of acute pulmonary thromboembolism in Japan have been focused on its early diagnosis and the rescue of the patients. However, the recent trends in the trials in the courts or the headlines in the newspapers have indicated that the major public interests are directed to its primary prophylaxis. Thus, the guidelines for the prophylactic strategy of acute pulmonary thromboembolism are needed today. It would be important to know (1) to what extent the thrombotic tendency should be examined, (2) how the patients with the risk could be identified, (3) which prophylactic interventions should be selected, and finally (4) what are the expected cost-benefit effects. An emphasis should be put on that the researches to derive quantitative estimates of the risks of thromboembolism and the studies of the efficacy of each of the prophylactic interventions in Japanese patients would provide the basis of such guidelines.