The longitudinal fracture toughnesses of human cortical bone were compared to those of bovine cortical bone to test the hypothesis that although human osteonal bone is significantly weaker and more compliant than primary (plexiform) bone, it is not less tough than primary bone. The fracture toughness indices, critical strain energy release rate (G_c) and critical stress intensity factor (K_c), were determined for human Haversian bone and bovine bone under tension (Mode I) loading using the compact tension method. The effects of thickness, crack growth range and anisotropy on fracture indices for slow stable crack growth in cortical bone were determined. Plane strain assumptions required for application of linear elastic fracture mechanics (LEFM) to bone were investigated. Longitudinal oriented fracture toughness tests were used to assess the crack inhibiting effect of human bone microstructure on fracture resistance. Human bone K_c calculated from the stress concentration formula for 2 and 3 mm thick specimens equaled 4.32 and 4.05 MN m^-3/2, respectively. Human bone G_c calculated from the compliance method equaled 827 N m^-1 for 2 mm thick specimens and 595 N m^-1 for 3 mm thick specimens. It was found that crack growth range, thickness and material assumptions affect fracture toughness. K_c calculated from Gc using an anisotropic relation provided the lowest estimate of K_c and equaled 3.31 MN m^-3/2 for 2 mm thick specimens and 2.81 MN m^-3/2 for 3 mm thick specimens. Both K_c and G_c were significantly reduced after being adjusted to ASTM standard thickness using ratios determined from bovine bone. The fracture toughness of bovine bone relative to human bone ranged from 1.08 to 1.66. This was compared to the longitudinal strength of bovine bone relative to the longitudinal strength of human bone which is approximately equal to 1.5. We found that even though human bone is significantly weaker than bovine bone, relative to its strength, the toughness of human and bovine bone are roughly similar, but the data were not sufficiently definitive to answer the question of which is tougher.