A 41-yr-old patient experienced an anaphylactic shock reaction caused by formaldehyde in a root canal sealant during endodontic treatment. The clinical events, positive skin tests, and a high level of immunoglobin E to formalin RAST (class 4) suggest the involvement of immunoglobin E-dependent mechanisms toward formaldehyde. This very infrequent observation in endodontic therapy focuses attention on the different pathological manifestations related to formalin, their mechanisms, and the prevention possibilities in dentistry.