Examination of totally or partially obstructed human aortocoronary vein grafts, obtained at different time intervals after the bypass operation, has shown that the initial occlusive process is due to thrombosis and may appear a few hours or days after surgery. The cellular phase of intimal proliferation affecting most of the grafts becomes apparent about 4 weeks after the operation. After 1 year the intimal hyperplasia acquires a cell-poor, fibrotic character; the graft usually remains patent. About one-half of the vein grafts obtained 3 or more years after the operation show complicated atherosclerotic lesions. These findings indicate that most of the vein grafts undergo extensive structural changes and some may show similar degenerative lesions as they develop in the coronary arteries.