The role of allograft veins in vascular reconstruction remains ill defined. The present experiment was undertaken to evaluate the role of immunosuppression in maintaining allograft patency in the canine femoral venous circulation. Twenty-seven mongrel dogs had segments of both femoral veins excised and each dog received one allograft and one autograft. The dogs were randomly assigned to a control group or to one of three treatment regimens of azathioprine. Low doses of azathioprine were of no benefit in improving patency of venous alografts. Microscopic evaluation of these grafts suggests that substantial intimal repopulation by host cells occurs by six to eight weeks in the canine model. Other methods of preserving patency until intimal repopulation occurs deserve further investigation.