A 50-year-old woman developed anaphylaxis 8 h after application of a paraformaldehyde-containing root canal disinfectant. Radioallergosorbent test showed that she had a high level of formaldehyde-specific IgE in her serum. Prick tests to formaldehyde and paraformaldehyde showed immediate-type responses to both. We reviewed the literature describing cases with anaphylaxis/angioedma caused by formaldehyde in root canal disinfectants and found that about 1/2 of the reported cases developed symptoms over 2 h after dental treatment. We speculated that the delay in the manifestation of her symptoms was possibly due to gradual formaldehyde release from paraformaldehyde and time lag of penetrating and diffusing of formaldehyde outside the dentin. Patch testing showed that she also had delayed-type allergy to formaldehyde, paraformaldehyde and eugenol. Physicians should pay attention to root canal disinfectants, even if anaphylaxis occurs several hours after dental treatment.