During a routine dental appointment, a patient had an anaphylactic reaction to an injection of a local anesthetic, mepivacaine HCl 3% without a vasoconstrictor. Immediate medical treatment alleviated the symptoms and prevented a more profound collapse. The patient may have been sensitized during emergency treatment at a hospital at which time a local anesthetic was probably administered. Keeping life-saving drugs on hand will help in the immediate treatment of patients undergoing anaphylactic shock, but taking an accurate medical history will provide a clue to a patient's previous exposure and reaction to drugs, thereby preventing a medical emergency.