BACKGROUND The purpose of this study was to quantitatively evaluate cortical bone thickness in various locations in the maxilla and the mandible. In addition, the distances from intercortical bone surface to root surface, and distances between the roots of premolars and molars were also measured to determine the acceptable length and diameter of the miniscrew for anchorage during orthodontic treatment. METHODS Three-dimensional computed tomographic images were reconstructed for 10 patients. Cortical bone thicknesses were measured in the buccal and lingual regions mesial and distal to the first molar, distal to the second molar, and in the premaxillary region at 2 different levels. Differences in cortical bone thickness at 3 angles (30 degrees, 45 degrees, and 90 degrees) were also assessed. Distances of the intercortical bone surface to the root surface and the root proximity were also measured at the above areas. RESULTS Significantly less cortical bone thickness was observed at the buccal region distal to the second molar compared with other areas in the maxilla. Significantly more cortical bone was observed on the lingual side of the second molar compared with the buccal side. In the mandible, mesial and distal to the second molar, significantly more cortical bone was observed compared with the maxilla. Furthermore, significantly more cortical bone was observed at the anterior nasal spine level than at Point A in the premaxillary region. Cortical bone thickness resulted in approximately 1.5 times as much at 30 degrees compared with 90 degrees Significantly more distance from the intercortical bone surface to the root surface was observed at the lingual region than at the buccal region mesial to the first molar. At the distal of the first mandibular molar, significantly more distance was observed compared to that in the mesial, and also compared with both distal and mesial in the maxillary first molar. There was significantly more distance in root proximity in the mesial area than in distal area at the first molar, and significantly more distance was observed at the occlusal level than at the apical level. CONCLUSIONS These data show that the safest location for placing miniscrews might be mesial or distal to the first molar, and an acceptable size of the miniscrew is less than approximately 1.5 mm in diameter and approximately 6 to 8 mm in length.