A morphologic trait, not previously registered, on the incisor lingual surface: "The lingual angle of incisors", was studied in Norwegians. The lingual angle is the angle between the floor of the lingual fossa and the dental tubercle. Measurements of the lingual angle could be performed with sufficient accuracy. Deviations from normal distribution in skewness and kurtosis values did not affect statistical testing. The general similarity of the frequency polygons for all incisors in the way they were peaked and stepped is possibly due to participation of the same factors on the genetic level in the determination of the trait. The mean lingual angles were (in degrees): I1sup = 142, I2sup equal 156, I1inf = 153 and I2inf = 154. No significant bilateral asymmetry or sex differences were found. Correlations were high for both central to central and lateral to lateral incisors in both jaws, highest for centrals in the maxilla and laterals in the mandibula. Central to lateral correlations were relatively low, especially in the upper jaw. Interjaw correlations were low, but I1sup showed a tendency to be better correlated than I2sup to lower incisors. I2sup and I1inf were the most variable incisors in each jaw. The dental tubercle probably plays an important role in determining the lingual angle. Measurements of the lingual angle give information on the profile of the lingual surface in inciso-cervical direction. Statistical analysis of a phenotypic trait may give information on factors which determine the trait.