We have one comment about the interesting report by Kim et al.1 on the interpretation of contact allergy to gold in their study. The authors state that, “...common allergens in the present study were gold sodium sulfate, nickel sulfate, and potassium dichromate1.” They focus their discussion on “the increasing demand by dental patients for precious metal alloys and tooth-colored restoration rather than amalgam1.” Although type IV hypersensitivity reaction to gold seems to have increased in recent years, however, patch testing is the most sensitive diagnostic test but is not specific for gold. In fact, gold cross-react with mercury in both humans and experimental animals234, a cross-reaction is likely to be the cause of about 10%~20% of the reactions reported to gold (Nakada et al.4 and our unpublished study). For quantitative analysis, atomic electronic structures of gold and mercury are very similar and very close in the periodic table, leading to an atomic mimicry by cross-reactivity between mercury and gold4. Of note, mercury has atomic number 80 and gold, with the slightly less atomic number (Z=79), do share their electrons giving stable metallic bond. Likewise, in type III hypersensitivity reactions (or Arthus-type reactions), it is interesting to note that both gold and mercury may induce pathological processes mediated by circulating immune complexes5. It may therefore be worth considering this possibility when examining patients with contact allergy to gold and cross-reactions between gold and mercury should not be forgotten.