We examined the process of vessel wall thickening in angiographically normal saphenous vein grafts (SVGs) using intravascular ultrasound. Fifteen SVGs were studied in the early stage (within 1 month postoperatively) and 14 SVGs in the late stage (over 6 months postoperatively). Lumen cross-sectional area (CSA) and vessel CSA were measured. Vessel wall area (VWA) was calculated and %VWA was defined as VWA / vessel CSA. Vessel CSA, VWA, and %VWA were significantly larger in the late stage than in the early stage (28.8 vs 21.6 mm(2), 15.8 vs 5.3 mm(2), 55.7% vs 24.9%, respectively) and lumen CSA was smaller in the late stage (12.8 vs 16.2 mm(2)). VWA correlated with vessel CSA, but not with lumen CSA. The time course of %VWA showed that %VWA in the late stage was a plateau state. From these findings, we concluded that the wall thickening process in SVGs begins within 6 months postoperatively and is accompanied by compensatory vessel enlargement.