Transgenic Populus trichocarpa wood was compared to the corresponding wild-type material. The static modulus of elasticity in three-point bending was measured and the chemical composition among the specimens were compared, including the glucose, xylose, and lignin contents as well as the S/G ratio. Changes in chemical composition, created by genetic manipulations of the lignin biosynthetic pathway, affect the mechanical properties of young small-diameter transgenic trees. There are indications that a decrease in lignin content causes severe reductions in mechanical properties. Changes in lignin structure, either from an increased S/G ratio or structural lignin modifications, also negatively influence the mechanical properties.