Diabetes mellitus is associated with depression of natural defenses against infection and increases the risk of periodontal disease. However, the effects of diabetes on periradicular tissue, which differs structurally from periodontal tissue, are not known. In this study, we evaluated the effects of type 2 diabetes on the development of periradicular lesions after exposure of the pulp in the left mandibular first molar through the occlusal surface in rats. GK rats with spontaneous non-insulin-dependent diabetes mellitus and Wistar rats (controls) received a normal laboratory diet and either water or a 30% sucrose solution. At both 2 and 4 weeks after pulp exposure, histologic analysis showed that alveolar bone resorption was most severe and the periradicular lesions were largest in diabetic rats given the sucrose solution. These results suggest that the metabolic conditions produced by type 2 diabetes enhance the development of periradicular lesions in rats.