Context: Bullous allergic reactions are inflammatory skin disorders, presenting usually as a result of some type of reaction to medication. Case Report: A 67-year-old female was evaluated for the presence of diffuse patches of erythema, microvesiculation, vesicles, crusts, and oozing of sudden appearance on the extremities and on the rest of the body after taking sulfamethoxazole in combination with trimethoprim. Skin biopsies for hematoxylin and eosin examination, as well as for direct immunofluorescence and immunohistochemistry analysis were performed. H&E staining demonstrated classic features. Direct immunofluorescence revealed strong deposits of fibrinogen in the vessels of the skin. The immunohistochemistry stain showed strong positivity of myeloperoxidase within the blister cavity. The distribution of the vessels around the inflammatory process were noticed by using antibodies to CD34 as well to collagen IV. Conclusions: sulfamethoxazole is catalysed by CYP2C9 and/or myeloperoxidase. Thus, myeloperoxidase appears to be of importance in this disorder.