A 67-year-old patient developed systemic reactions after application of a formaldehyde-containing tooth filling. She had a clearly positive RAST result for formaldehyde, whereas skin prick testing and patch tests were negative. Sensitization to formaldehyde appears to have occurred 1 year previously. Induction of formaldehyde allergy may represent a major complication during dental treatment, and assessment of specific IgE should be considered in patients at risk.