Increasing recognition of the importance of vein graft stenoses in precipitating failure of femorodistal bypass procedures has stimulated an increasing interest in noninvasive postoperative surveillance. We have used duplex scanning, measuring relative changes in velocity throughout the entire length of the graft, to detect nonhemodynamic stenoses (i.e., stenoses without a significant change in ankle-brachial pressure indexes) as well as more severe lesions during the postoperative period. Seventy-five in situ vein grafts were assessed at three monthly intervals from operation with duplex scanning and intravenous digital subtraction angiography. Nineteen grafts (25%) had angiographically documented stenoses at a mean follow-up of 12 months. All 19 stenoses were detected independently by duplex velocity ratio criteria and 15 were correctly graded as mild, moderate, or severe. The results suggest that duplex-derived velocity ratio criteria are appropriate for graft surveillance and for determining the natural history of even minor stenoses to identify the optimal time of surgical intervention.