The induction of sensitivity to various sulfonamide compounds and the possible physiologic mechanisms responsible are subjects of increasing interest and importance. This paper is a report of a reaction which we have observed in 12 patients, in whom a sensitivity to sulfathiazole was apparently induced by local application of it to diseased skin, and in whom the evidence of sensitivity appeared when the drug was later administered by mouth. These patients all suffered from various forms of eczematous dermatitis complicated by some degree of low grade secondary pyococcic infection and were treated for more than five days with 5 per cent sulfathiazole emulsion type ointment and later given sulfathiazole by mouth. A characteristic explosive type of systemic and dermatologic reaction occurred. These patients were part of a series of more than 1,000 patients suffering from various dermatoses whom we have treated with the sulfonamide compounds applied locally.The several reports regarding the use of sulfathiazole ointment in the treatment of cutaneous pyogenic infections will not be completely summarized here.1In three reports some local reaction to sulfathiazole ointment was reported. Sams and Capland 2 observed a patient in whom the local application of sulfathiazole powder for chronic otitis externa produced an acute exacerbation. Later administration of sulfathiazole by mouth to this patient was followed by massive edema of the face and ears and a generalized erythematous macular and pustular eruption. Recently Miller3 reported that a patient with sycosis vulgaris had been given applications of 50 per cent and 25 per cent sulfathiazole ointment for a total of two weeks. Two months later the patient was given 1 Gm. of sulfathia¬ zole by mouth and within two hours the eyelids became swollen shut and an eruption appeared on the face, forearms, back, abdomen and thighs. In another study we4 reported on the treatment of 190 patients with sulfathiazole incorporated in bases of various types. In this paper we stated that "in a few patients with chronic eczematoid eruptions, flare-ups have been noted in conjunction with the use of sulfathiazole ointment but no exacerbation has been noted in a patient with a dermatosis primarily due to infection." None of the patients in this series had been given sulfathiazole bymouth after local application of the drug. It was demonstrated that sulfathiazole ointment (particularly when the ointment base is of the emulsion, water solu¬ ble type) is an extremely effective therapeutic agent in the treatment of impetigo, ecthyma and acute impetiginous dermatitis uncomplicated by other etiologic factors. However, it was emphasized that sulfathiazole ointment has a much less striking effect on eczematous lesions complicated by chronic bacterial infection than on acute pyodermas due solely to pyococci. Our sub¬ sequent experience in a large series of patients is in accord with this. In addition, certain limitations and contraindications to sulfathiazole ointment therapy have become apparent, and...