Hypersensitivity reactions to erythritol are rare, with an incidence of less than 1:1,000,000. In recent years, the widespread use of erythritol has increased significantly, which may result in an increase of the frequency of hypersensitivity reactions. We discuss the clinical features and the diagnostic process in a case of hypersensitivity to erythritol in a 7-year-old boy. The presented patient developed generalised urticaria, vomiting, weakness, and shortness of breath within minutes of eating a homemade cake containing erythritol. A few months later urticaria appeared on the boy’s face and trunk 30 minutes after eating a homemade cake containing maltitol. Skin prick tests and intradermal tests with erythritol were performed, yielding negative results. An oral challenge with erythritol, using increasing doses of 250 mg, 500 mg, and 1,000 mg every 30 minutes, was positive. It was recommended to eliminate sweeteners from the diet. It is important to remember about food additives when searching for the causative agents of anaphylactic reactions.