Adverse reactions to phenol-formaldehyde resins include depigmentation, irritant dermatitis, chemical burns and allergic contact dermatitis. Allergic contact dermatitis from phenol-formaldehyde resin has mainly been ascribed to resins based on paratertiary-butyl phenol and formaldehyde, and such a resin is included in the ICDRG standard patch test series. When 1220 patients were patch tested with this resin as well as with 2 other phenol-formaldehyde resins, based on phenol and formaldehyde, 26 patients were positive to at least 1 resin. The figures for positive reactions to paratertiary-butyl phenol-formaldehyde resin and the 2 other resins were 0.8%, 1.0% and 3.0% (440 tested subjects), respectively. Therefore, a battery of phenol-formaldehyde resins should be used for screening purposes, since patch testing with the paratertiary-butyl phenol-formaldehyde resin is not sufficient to identify patients with contact allergy to phenol-formaldehyde resins. Several of the 26 patients were patch tested with the basic substances phenol, formaldehyde and paratertiary-butyl phenol, but only 1 positive reaction to formaldehyde was noted. The sensitizing capacity of 2-methylol phenol, 4-methylol phenol and 2,4,6-trimethylol phenol, all 3 compounds being possible ingredients of resins based on phenol and formaldehyde, was demonstrated; 5 of 14 resin positive patients reacted to at least 1 of these methylol phenols.