In an open multicenter comparative study aimed at the evaluation of the efficacy of defibrotide in the prophylaxis of postsurgical deep vein thromboses (DVT) an ad interim evaluation has been made on 2626 patients thus far enrolled. 1323 had received defibrotide (200 mg q.i.d. by IV route from day -1 to day +7th postoperative), 941 calcium heparin (5000 IU b.i.d. or t.i.d. by SC route from day 0 to day +7 postoperative) and 362 other treatments (antiaggregating agents, placebo or no therapy). This group has not been included in the final evaluation, due to its limited size. The diagnosis of DVT or pulmonary embolism (PE) was made according to clinical routinary criteria. The incidence of DTV has been 15/1323 (1.13%) in the defibrotide group and 21/941 (2.23%) in the heparin group (chi-square, p = 0.056) while the cases of suspected or ascertained PE have been respectively 3/1323 (0.22%) and 10/941 (1.06%) (p = 0.02). The incidence of adverse effects with defibrotide was less than 1%; occasional cases of increased serum transaminase levels were seen with heparin. These preliminary results supports the effectiveness of defibrotide in the prevention of post-surgery DVT, its effects being similar or more prominent than those of calcium heparin, currently regarded as the standard medication.