Vein wall morphology (intimal and medial thickness) was studied in a consecutive series of 100 veins before organ culture. The development of a proliferative neointima after 14 days in culture was quantified by measuring neointimal thickness, and the relationship between this and the original vein wall morphology was investigated. The correlation between the intimal or medial thickness before culture and neointimal thickness following culture was poor (rS intima/neointima = 0·21, rS media/neointima = 0·09). Long saphenous vein biopsies were obtained from 53 patients at the time of infrainguinal bypass surgery and their morphology was studied as above. Median intimal and medial thickness before grafting was 21 (range 2–208) and 245 (range 108–557) μm respectively. After 12 months in a colour duplex graft surveillance programme, 46 grafts were patent and 17 of these had developed a stenosis. There was no difference in intimal (28 versus 19 μm, P = 0·1) or medial (247 versus 245 μm, P = 0·47) thickness between stenosed and non-stenosed grafts respectively. Both the laboratory and clinical evidence from this study fail to support the hypothesis that pre-existing vein wall thickening is a key factor in the aetiology of vein graft stenosis.