OBJECTIVE: Until now, no study established a morphometric evaluation of the truncal valve dysplasia and a description of its different presentation patterns. Thus, authors conducted an anatomopathological study describing the gross features and histological findings of the truncal valve. METHODS: 50 common arterial trunk (CAT) specimens were examined. The number of valvar leaflets was determined and valvar dysplasia was classified as absent, mild, moderate or severe. Selected leaflets were sectioned and submitted to histological analysis and linear measurements (thickness, length and area), besides quantification of collagen area fraction. RESULTS: 28 (56%) valves presented three, 15 (30%) four and 7 (14%) two leaflets. Valvar dysplasia was absent in 13 (26%) cases, mild in 19 (38%), moderate in 6 (12%) and severe in 12 (24%). A significant association was found between the presence of four leaflets and valvar dysplasia (p<0.001). Single coronary ostium was more common in two-leaflet cases than in three-leaflet cases (p=0.037). Leaflets medial thirds were thicker in the more dysplastic valves (p=0.006) and in those presenting anarchic collagen distribution (p=0.002). CONCLUSIONS: CAT semilunar valves present two main patterns. The first characterised by three leaflets and absent or mild dysplasia and the second by four leaflets and severe dysplasia. Still, great variability regarding thickness, microscopic organisation of the extracellular matrix and proportions of leaflets' dimensions exists, which may impact on the surgical outcomes.