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The purpose of this study was to investigate whether individual variation in the hardness and chemical composition of the cementum in the root apex affects the degree of root resorption.In a previous study, we evaluated the Vickers hardness scale of 50 extracted teeth. For this study, we classified the 50 extracted teeth into soft, moderate, and hard groups according to the Vickers hardness scale. Then, we randomly selected 7 teeth from each group and measured the resorbed areas of the apical cementum in vitro using human osteoclast precursor cells. We also investigated the calcium/phosphorous (Ca/P) and magnesium/calcium ratios of these 21 extracted teeth using energy-dispersive x-ray microanalysis studies to determine the chemical composition of the cementum in the root apex.In the pit formation assay, the resorbed area in the soft group showed a greater extent than it did in the moderate and hard groups (P < 0.01). A correlation was noted between the Vickers hardness and the resorbed area of the cementum in the root apex (r = −0.714; P < 0.01). The Ca/P ratios in the soft and moderate groups were lower than the ratio in the hard group (P < 0.01 and P < 0.05, respectively). A correlation was noted between the Vickers hardness and the Ca/P ratio of the cementum in the root apex (r = 0.741; P < 0.01).These results suggest that the hardness and Ca/P ratio of the cementum may be involved in root resorption caused by orthodontic forces.Hardness and chemical composition were assayed in the cementum of extracted teeth.The resorbed area was greater in the soft group than in the moderate and hard groups.Ca/P ratio of the apical cementum was lower in the soft and moderate groups.Hardness and Ca/P ratio of the cementum may be involved in root resorption.