This is one of series of review articles on formaldehyde and formaldehyde-releasers (others: formaldehyde in cosmetics, in clothes and in metalworking fluids and miscellaneous). Thirty-five chemicals were identified as being formaldehyde-releasers. Although a further seven are listed in the literature as formaldehyde-releasers, data are inadequate to consider them as such beyond doubt. Several (nomenclature) mistakes and outdated information are discussed. Formaldehyde and formaldehyde allergy are reviewed: applications, exposure scenarios, legislation, patch testing problems, frequency of sensitization, relevance of positive patch test reactions, clinical pattern of allergic contact dermatitis from formaldehyde, prognosis, threshold for elicitation of allergic contact dermatitis, analytical tests to determine formaldehyde in products and frequency of exposure to formaldehyde and releasers. The frequency of contact allergy to formaldehyde is consistently higher in the USA (8-9%) than in Europe (2-3%). Patch testing with formaldehyde is problematic; the currently used 1% solution may result in both false-positive and false-negative (up to 40%) reactions. Determining the relevance of patch test reactions is often challenging. What concentration of formaldehyde is safe for sensitive patients remains unknown. Levels of 200-300 p.p.m. free formaldehyde in cosmetic products have been shown to induce dermatitis from short-term use on normal skin. Formaldehyde is a common cause of contact allergy. In Europe, 2-3% of patients suspected of contact dermatitis have positive patch test reactions, and in the USA prevalence rates of sensitization of 8-9% are reported in this selected group of patients. Allergic contact dermatitis caused by formaldehyde is often chronic, presumably because it is difficult to avoid exposure to the allergen completely. Indeed, formaldehyde may be found in many cosmetics, toiletries, household products such as washing and cleaning agents and in a great number of industrial applications including adhesives, paints, lacquers and metalworking fluids. Often, the products are not preserved with formaldehyde itself, but with agents that release formaldehyde under usage conditions, the so-called formaldehydereleasers (or formaldehyde donors). Well-known examples are quaternium-15, imidazolidinyl urea, diazolidinyl urea, DMDM hydantoin and 2-bromo-2-nitropropane-1,3-diol, preservatives frequently used in cosmetic products. Industrial products such as metalworking fluids frequently contain formaldehyde donors, such as the Bioban ® product range of biocides and tris(N-hydroxyethyl) hexahydrotriazine (better known by its trade name Grotan ® BK).Other products containing and releasing formaldehyde are the formaldehyde resins including urea formaldehyde and melamine formaldehyde resins. These were formerly used extensively as textile