The posterior cusp of the left atrioventricular valve was studied in 100 cadavers of Brazilians. There were 58 Caucasians and 42 non-Caucasians, 65 males and 35 females. Their age ranged from 18 to 72 years. In 91% of the cases the posterior cusp had 3 leaflets (posteromedial or right, intermediate, and anterolateral or left). The intermediate leaflet was wider (mean 18.8mm) and higher (mean 12.7mm) than the other two. All leaflets of the posterior cusp were shorter than the anterior cusp, a feature that confers on the posterior cusp the role of a supporting structure in addition to its role in the completion of closure of the left atrioventricular ostium.