In an open multicenter comparative study aimed at the evaluation of the efficacy of defibrotide and calcium heparin in the prophylaxis of postoperative deep vein thrombosis (DVT) and pulmonary embolism (PE) an evaluation has been made on 4810 patients. 2810 patients received defibrotide (800 mg/die: 400 mg b.i.d. or 200 mg q.i.d. by IV route); 2000 patients received calcium heparin (500 IU b.i.d. or t.i.d. by SC route) till the 7th post-operative day. The diagnosis of DVT or pulmonary embolism (PE) was made according to clinical routine criteria. The incidence of DVT has been 33/2810 (1.17%) in the defibrotide group and 47/2000 (2.35%) in the heparin group (chi 2 p = 0.002), while the cases of suspected or ascertained PE have been respectively 15/2810 (0.53%) and 23/2000 (1.15%) (p = 0.025). The post-operative course was totally normal in both groups; no significant adverse reactions were noticed. The obtained results suggest the effectiveness of defibrotide as an alternative to heparin in the prevention of post-operative DVT.