Torsion at the microanastomosis site is a basic fault and should be avoided. In this study, we investigate the effects of different degrees of microvenous torsion on patency and its physical changes on anastomoses in a rat model. One hundred anastomoses were performed at different degrees of torsion, using femoral veins of Sprague-Dawley rats. Anastomoses were performed at 0degrees, 451, 90degrees, 1350, and 180degrees of torsion randomly. Patency tests immediately, 1 h, and 1 week after the anastomoses were checked, using the refill test. Measurements of external diameter were recorded at three points: one at the anastomosis site, and the others 2 mm proximal and distal to the anastomotic site. Finally, histopathologic and scanning electron microscopy studies were performed. Subsequently, because of the peculiar phenomenon of early recannulation of the thrombosed vessels, 20 vessels were also explored on the first and the third days postoperatively. The data demonstrate that torsion at 180degrees, compared with 0degrees, 45degrees, and 90degrees, impaired patency significantly (P < 0.005). In the subsequent study of 20 veins that were thrombosed on the first day, all became patent on the third day and remained so. In conclusion, rotation of a microvenous anastomosis begins to affect the patency rate at 90degrees of torsion, and at 180degrees has a patency rate of only 25%. However, all become patent again from the third day onwards. Thrombosis of rat femoral veins without chronic obstruction results in rapid lysis of thrombus and transient proliferative changes. (C) 2003 Wiley-Liss, Inc.