Patients undergoing dental treatment can be exposed to a wide range of potential allergens, but adverse events seem infrequent. Patients with symptoms or signs of stomatitis, burning, tingling, cheilitis, oral lichenoid lesions, lip and facial swelling may relate their problems to dental treatment or to the use of dental products. Investigation for immediate type or delayed type hypersensitivity is indicated using patch testing, prick testing and blood tests for allergen-specific IgE. The main allergic reactions found in patients include contact allergy to metals, cosmetics, food additives, flavours and acrylates, and immediate type allergy to latex. Adverse reactions following the administration of local anaesthetics are seen in about 0·5% of cases, but immediate type allergy to these agents is rare. In dental staff, occupationally related problems are common and usually take the form of hand or facial dermatitis or respiratory disease. The most common allergic reactions in dental staff are immediate type allergy to latex, and contact allergy to rubber additives, fragrances, acrylates and formaldehyde. Occupational irritant problems causing hand dermatitis are probably more common in dental personnel than is dermatitis caused by contact allergy. Patch testing and tests for immediate type allergy are useful investigative methods in the investigation of patients who present with oral or facial symptoms possibly related to dental treatments and are also beneficial in dental personnel who present with hand or facial dermatitis or respiratory symptoms.