Dermatitis herpetiformis (DH) of linear IgA type occurred in a 6-month-old boy shortly after initiating sulfisoxazole therapy for a urinary tract infection. Generalized pruritic bullae on erythematous bases were present on his trunk and extremities. There were no clinical or laboratory findings suggestive of gastrointestinal involvement. Direct immunofluorescent studies of skin biopsies taken early in the course of the disease and while the child was on systemic corticosteroid therapy were negative. Eventually a linear deposition of IgA at the dermoepidermal junction of involved skin on direct immunofluorescence was demonstrated. No circulating antibodies to the basement membrane were found. Because of close proximity of the initiation of sulfisoxazole (Gantrisin) therapy and the eruption of the initial bullous lesions, this case also presents an interesting diagnostic and therapeutic problem. Negative assays of lymphocyte migration inhibition factor (LMIF) to sulfisoxazole indicated that the likelihood of a hypersensitivity reaction to sulfa drugs was slight. The patient's clinical response to dapsone therapy was dramatic. The conflicting views of subepidermal bullous dermatosis of childhood and the difficulties in confirming a diagnosis of DH are discussed. We contend that when DH is suspected in children, various laboratory tests should be repeated several times before the diagnosis can be confirmed. The case presented here is the youngest child reported with this type of DH.