An autopsy study was conducted of 14 cardiopulmonary specimens in which porcine-valved extracardiac conduits had been placed from the right ventricle to the pulmonary artery. The gross and light microscopic appearance of the tissue lining in such conduits was evaluated. In no instance was the lining considered obstructive. The conduits had been in place for varying lengths of time ranging from 1 day to 5 years. The neointima appeared to begin developing as early as the first postoperative day as a thin layer of platelet-fibrin thrombus, followed by incorporation of red blood cells and a few leukocytes. Between 2 to 3 weeks postoperatively, such shallow thrombotic linings had become organized by fibroblastic migration and proliferation from both the proximal and distal anastomosis sites. However, incomplete development of the neointima frequently led to fenestrations. Peels, or neointimae, older than 1 month did not change, having densely fibrous luminal surfaces and having thrombus or necrotic thrombotic debris along the interface between the conduit and peel. These findings support the concept that progressive thickening of the lining peel takes place primarily along the interface between the peel and the conduit rather than the interface between the peel and the lumen.