We hypothesized that recently formed, incompletely mineralized, and thus, relatively deformable osteons in the equine third metacarpus enhance in vitro load-controlled fatigue life in two ways. Macroscopically, there is a compliance effect, because reduced tissue elastic modulus diminishes the stress required to reach a given strain. Microscopically, there is a cement line effect, in which new osteons and their cement lines more effectively serve as barriers to crack propagation. We studied 18 4×10×100 mm beams from the medial, lateral, and dorsal cortices of metacarpal bones from 6 thoroughbred racehorses. Following load-controlled fatigue testing to fracture in 4 point bending, a transverse, 100 μm thick, basic fuchsin-stained cross-section was taken from the load-bearing region. The number and diameter of all intact (and thus recently formed/compliant) secondary osteons in a 3.8×3.8 mm region in the center of the section were determined. The associated area fraction and cement line length of intact osteons were calculated, and the relationships between these variables, elastic modulus ( E), and the logarithm of fatigue life (log N F) were analyzed. As expected, log N F was negatively correlated with E, which was in turn negatively correlated with intact osteon area fraction and density. (Log N F)/E increased in proportion to intact osteon density and nonlinearly with cement line density (mm/mm 2). These results support the hypothesis that remodeling extends load-controlled fatigue life both through the creation of osteonal barriers to microdamage propagation and modulus reduction.