Seven patients are described, who presented with a history of recurrent episodes of erythema multiforme and who were subsequently found to be sensitive, by patch testing, to benzoic acid. Advice on avoidance of benzoic acid resulted in resolution of attacks in four patients, but three patients who were not able to adhere to an exclusion diet continued to have symptoms. These findings implicate benzoic acid, a food preservative, as an aetiological factor in erythema multiforme. It is proposed that patch testing to food substances should be carried out on patients who present with recurrent erythema multiforme, since avoidance of potential allergens may lead to resolution of symptoms.