One of the characteristics of diabetes mellitus is the exaggerated inflammatory response. The present report shows the reaction from the use of a rapid maxillary expander in a diabetic patient. A 9-year-old child presented an uncommon reaction to the treatment with a rapid maxillary expander, and on follow-up examination, it was discovered that the patient had diabetes mellitus. After controlling the disease, the proposed treatment was used without further incidents. The case calls attention to the presence of uncommon responses to treatment and the need for the orthodontist to suspect a patient's systemic compromise. (Angle Orthod. 2011;81:546-550.)