The potassium iodide patch test was studied in twenty-six patients with dermatitis herpetiformis. Histological assessment was found more sensitive than clinical. All of five patients with active disease and not on treatment had a positive test, whereas only two of six patients taking a gluten-free diet (GFD) and one of eight taking dapsone were positive. In another two patients taking a GFD, but in whom the diet had not been strict, the test was positive. All three patients in remission and both patients with the linear pattern of IgA (but with active disease) were negative. Immunofluorescence studies showed no difference in the presence, quantity, or distribution of immunoglobulin, complement or fibrinogen between the patch test site and uninvolved skin, or in the uninvolved skin between patients with and without active lesions.