Postoperative thromboembolic complications do present an underestimated problem whenever their detection simply relies upon individual clinical judgement. Major abdominal operations are at increased risk of pulmonary embolism (PE) and deep vein thrombosis (DVT), mostly in advanced age, overweight subject, and in patients with cardiac or malignant diseases, or with previous venous diseases. Such patients may benefit from a peri- and postoperative prophylaxis with chemical or mechanical procedures, as a recent meta-analysis seems to suggest. In our experience, a randomized, multicentric clinical trial with defibrotide (DF) versus calcium-heparin (CH) was realized with the aim of evaluating their effectiveness and side effects in the prophylaxis of PE and DVT after major abdominal surgery; 1296 patients were randomly assigned to i.v. DF (400 mg.) or subcutaneous CH (0.2 ml. = 5000 U.I.) given one hour prior to operation and twice daily for seven days postoperatively. Definitive evaluation was carried out on 1212 patients (610 patients in DF and 602 in CH group, respectively) who completed the prophylaxis and monitoring schedule acceptably. One PE (0.16%) and 38 DVT (6.2%) were detected in DF group while 2 PE (0.33%) and 40 DVT (6.6%) were reported in CH treated patients. Post-operative blood loss was 578 +/- 150 cc. (median +/- S.E.M.) in DF group and 604 +/- 123 in CH group (p = n.s.). Wound hematoma was observed in 69 patients (5.7%): 20 (3.2%) in DF and 49 (8.1%) in CH group of patients (CHI-Square = 12.44; p = 0.0005); a significant post-operative decrease of RBC, WBC, Platelet count, and Fibrinogen was computed in both groups (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)