Increased bone fragility and osteoporotic fracture in human bone has been attributed to the accumulation of microdamage. According to Martin and Burr (J Biomech 15:137–139; 1982), microcracks from interstitial bone propagate to the cement line or concentric lamellae and debond or separate the Haversian canal from the surrounding bone which leads to repair of the damaged region. If this is true, we would expect to find a greater incidence of microdamage existing at the cement line than at other locations within the bone microstructure. The incidence of such an occurrence, however, is not established. The purpose of this investigation was to determine the incidence and morphology of microcracks in human cortical bone from the midshaft of the tibia and proximal femur. We investigated the hypothesis that osteons arrest and trap microcracks in bone. We were also interested in determining if relationships exist between microdamage and bone type (tibia or femur), cortex location (anterior, posterior, medial, and lateral), gender, and donor age. It was found that 62.4% of all microcracks run between the surrounding interstitial bone and the cement line supporting the hypothesis by Martin and Burr. It was also found that microdamage increased with age, was significantly greater in the females than males, and was significantly greater in the proximal femur than the midshaft of the tibia.