Stiffness of compact bone is found to be highly and nonlinearly dependent on its porosity, its complement, bone volume fraction and apparent density. Elastic modulus decreases as a power (0.55) of increasing porosity and increases both as a power of increasing bone tissue volume (10.92) and increasing apparent density (7.4). These data indicate that small changes in the amount or density of compact bone tissue exert a more pronounced influence on its stiffness than would similar changes in trabecular bone.