Tree roots markedly stabilize colluvium on steep hillsides in the Cincinnati area, Ohio, according to detailed analysis of a landslide complex on the south side of Rapid Run Creek, a tributary of the Ohio River. The shear surface of the slide developed near the interface between bouldery, silty clay colluvium and underlying calcareous Ordovician shale, and it broke woody roots that penetrated through the colluvium and into the underlying bedrock. Measurements of positions, sizes, and tensile strengths of the roots indicate that the average shear strength contributed by the roots was about 5.7 kN/m2 of the shear surface. The average strength contributed by residual friction of the soil was about 0.7 kN/m2. The tree roots increased the factor of safety against sliding nine-fold. Root strength allows forested, colluvium-mantled hillslopes in the Cincinnati area to resist sliding at slope angles as high as 35°, whereas similar slopes devoid of trees are subject to sliding at slope angles of 12° to 14°. The contribution of tree roots to slope stability should be evaluated before removal of trees for development of hillsides.