BACKGROUND - The use of saphenous vein grafts (SVG) in coronary artery bypass surgery is established but little is known of SVG remodeling during the first year in vivo. METHODS AND RESULTS - The feasibility of measuring total vessel diameter (lumen plus wall), lumen diameter, and wall thickness by a novel computed tomography (CT) method was established in phantom model tubes (r=0.98 for lumen diameter and r=0.98 for wall thickness) and in an initial clinical study of 14 patients correlating CT and intravascular ultrasound measurements of SVG (r=0.88 for total vessel diameter, r=0.85 for lumen diameter and r=0.89 for wall thickness). In a separate group of 42 patients (aged 66±10 years; 36 male, 6 female) undergoing coronary artery bypass grafting, SVG total vessel diameter, lumen diameter, and wall thickness were determined prospectively with multi-slice CT angiography at 1 and 12 months postoperatively. Mean total vessel diameter decreased from 5.95±0.83 mm to 5.39±0.87 mm, P<0.001 (range, -39% to +8% change). Twenty-six patients (62%) had a decrease of SVG vessel diameter (negative remodeling) >5%. Mean lumen diameter decreased from 3.69±0.66 mm to 3.36±0.68 mm, P<0.001, (range, -40 to +11% change). Surprisingly, mean wall thickness decreased from 1.14±0.27 mm to 1.01±0.21 mm (P<0.001; range, -48 to +33% change). CONCLUSIONS - Lumen loss in SVG between postoperative months 1 and 12 is predominantly caused by negative remodeling of the whole vessel rather than to changes in wall thickness. Therapies targeting negative remodeling may be required for optimal maintenance of SVG lumen in the first postoperative year.