This article reports on a 60-year-old woman who had an allergic reaction to methylmethacrylate self-curing acrylic resin during orthodontic treatment. A localized hypersensitive reaction appeared on the palate after an orthodontic retainer was placed. Samples of the acrylic were removed and analyzed with gas chromatography to evaluate the residual monomer level. The residual monomer content was between 0.745% and 0.78%, which did not exceed international standards for this material. Patch tests were performed with several methylmethacrylate resin samples and processed with various techniques; they showed positive reactions. Despite the many alternative products available, self-curing acrylic resin remains widely used because of its low cost, ease of use, and diversity of indications. Orthodontists should be aware that allergic reactions can occur.