Landform is related to environmental factors that affect site productivity in mountainous areas. I devised a simple index of landform and tested this index as a predictor of site index in the Blue Ridge physiographic province. The landform index is the mean of eight slope gradients from plot center to skyline. A preliminary test indicated that the index was significantly associated with slope position and three classes of landform (ridge, slope, and cove). In a test with data from four locations, site index of yellow-poplar (Liriodendrontulipifera L.) was significantly correlated with landform index for each location (r = 0.45–0.65). Landform index and two other topographic variables together accounted for 31 percent of the variation in yellow-poplar site index throughout the Blue Ridge province. Landform index is a conveniently measured site variable that may be useful in various forestry-related applications, including multivariate analysis of the distribution and composition of forest vegetation.