Intimal hyperplasia is one of the main risk factors for the patency of small diameter bypass grafts. The standard unfractioned heparin (UH) is able to control this phenomenon but the clinical use is not fit for long term treatment; on the other hand the antiplatelet drugs have an anti-thrombotic effect but they seem to be unable to control intimal hyperplasia. Low molecular weight heparins (LMWH) have an anti-thrombotic effect superimposable to that of UH with minimal side-effects and might inhibit intimal hyperplasia too. Based on these criteria, we carried out an experimental study on sheep with the aim of evaluating the efficacy of postoperative treatment with LMWH versus an anti-platelet drug in controlling intimal hyperplasia and growth of true endothelial cells in small prosthetic ePTFE grafts (4 mm) interposed in the carotid artery. At the operation, 30 sheep were randomly located in 3 groups: A = control group, no treatment; B = Ticlopidine hydrochloride 250 mg/bid by mouth for 4 weeks; C = LMWH 3.075 IU AXa (0.3 ml) sc preoperatively and then once a day for the same period. Complete thrombosis of the graft occurred in 7 sheep of group A, 5 in B and 2 partial in group C. The intimal hyperplasia was moderate-severe in group A, mild-moderate in group B and no-mild in group C. True endothelial cells were found mainly in the LMWH group; in the other groups and in other portions of the grafts the cellular coverage was accomplished almost completely by fibroblasts. The study is still in progress with 6 further sheep treated with LMWH.