The authors studied the effect of pentoxifylline (Trental), a hemorrheologic agent that increases erythrocyte flexibility and augments microcirculatory blood flow, on the patency of a known thrombosis model. In Sprague-Dawley rats, a double-blind study was performed, with animals receiving either intraperitoneal injections of pentoxifylline (20 mg/kg/day) or saline in an equivalent volume, 4 weeks prior to surgery and continued for 1 week postoperatively. In all animals, the left femoral artery was used for a 2-mm arterial inversion graft (AIG), and the contralateral femoral artery for a 2-mm crush injury model. In the control group, patency of the AIG was 0 percent (0/19) and that of the crush model 31 percent (6/19). The pentoxifylline-treated group showed an AIG patency of 37 percent (7/19) and a crush model patency of 84 percent (16/19). p values were .004 and .005, respectively. Furthermore, hematoma formation was associated with none of the control sides (0/38) and only with 5 percent (2/38) of the pentoxifylline-treated animals. This study demonstrated statistically significant improvement on microvascular patency in animals treated with pentoxifylline, whether they were in the crush or the arterial inversion graft groups. The clinical ramifications of using a hemorrheologic agent that improves microvascular circulation, without effecting changes in coagulation, are apparent.