Twenty three morphological specimens of truncus arteriosus were examined for evidence of stenosis of the semilunar valve. One third showed good evidence of stenosis as judged by careful measurement of the valve orifice, the valve ring, and the maximum diameter of the truncus. Correlation with measured pressure gradients was poor, but angiography and cross sectional echocardiography were better predictors of stenosis. Stenosis was invariably associated with cusp dysplasia and was more common in valves with two or four cusps.