The purpose of the present study was to document the mean proximal alveolar bone loss of molars. The samples consisted of 219 subjects receiving of full mouth radiographs by standardized paralleling technique from Jan, 1992 to Jun, 1994. All the radiographs of 219 individuals suffered from adult periodontitis at age between 20 and 65 years old were measured, and were assessed mean proximal alveolar bone loss of molars and associated contributing factors. The main results indicated that (1) the mean proximal alveolar bone loss of the maxillary first and second molars accounted for 38. 4% and 33.5%, respectively, whereas the mandibular first and second molars were 34.8% and 31.6%, respectively; (2) within the same dental arch, mean proximal alveolar bone loss of the first molars was significant greater than that of the second molars, while the bone loss in the maxillary first molars was significant greater than that of the mandibular first molars. There was no difference between mean proximal bone loss of the maxillary and mandibular second molars; (3) mean alveolar bone loss of the first molars was significant greater than second molars in the same side of the dental arch. There was no significant difference in the mean proximal alveolar bone loss between right and left side molars. (4) average bone loss was the greatest (39.4%) at the mesial surfaces of maxillary first molars, whereas the least mean alveolar bone loss appeared at the distal surfaces of mandibular second molars. A significant difference of mean proximal alveolar bone loss was found between mesial and distal surfaces in mandibular first molars.