Thirteen of 308 patients (4.2%), who had received right-sided valved extracardiac conduits at the Mayo Clinic from November, 1972, to April, 1977, have required conduit replacement because of obstruction. Patients were 5 to 16 years old at initial operation and 8 to 20 years old at reoperation; the duration of conduit implantation was 27 to 79 months (mean 50). Four patients (31%) were asymptomatic; exertional dyspnea was present in eight (62%) and dizziness was reported in one (8%). Signs of conduit obstruction included increasing intensity of murmurs in 11 (85%), cyanosis in two (15%), and heart failure in one (8%). Peak pressure gradients from the right ventricle to the pulmonary arteries ranged from 50 to 140 mm Hg (mean 87) and correlated well with the degree of conduit obstruction. Catheter pullback allowed accurate localization of stenosis within the conduit, whereas angiography alone did not. The site of major obstruction was in the proximal conduit in five (38%), at the valve in nine (69%), and in the distal conduit or side branches in six (46%); stated differently, major stenosis affected the valve alone in five (38%), the graft alone in four (31%), and both the valve and the graft in four (31%). Valvular changes leading to stenosis included thrombosis, commissural fusion, and calcification, and changes consistent with insufficiency included tears, fusion of cusps to the conduit wall, and, in one case, infective endocarditis. Within the conduit, nonvalvular obstruction was due to formation of a thick fibrous peel (or neointima). Progressive thickening of the peel appeared to be due to organization of thrombus between the peel and conduit and not due to luminal mural thrombus. In one case, the peel formed a flap-valve, causing even further obstruction. Since many patients are asymptomatic, and since late conduit stenosis may develop unpredictably by several mechanisms, long-term follow-up is necessary.