Ultraviolet light (UVL) applied to the normal appearing skin of patients with pemphigus vulgaris and bullous pemphigoid prior to systemic therapy produced fresh histologic and immunohistochemical changes in the irradiated skin. The immunologic changes leading to the development of those new lesions was a progressive increase in the in vivo binding of IgG and complement in the areas of the disease. This study demonstrates new evidence for the antigen-antibody nature of the in vivo bound IgG and the possible pathogenic role of complement in these diseases. Further, this simple UVL test provides a useful procedure to help confirm the diagnosis of pemphigus and bullous pemphigoid in certain patients.