BACKGROUND AND DESIGN: From previous studies, we concluded that the fluorescence overlay antigen mapping (FOAM) technique could be of value to the differential diagnosis of the acquired subepidermal bullous skin disorders, bullous pemphigoid (BP) and epidermolysis bullosa acquisita (EBA). In these diseases, ultrastructural identification of the site of skin-bound IgG deposits at the epidermal basement membrane zone (EBMZ) may be essential to the correct diagnosis. Since ultrastructural studies are more expensive, time-consuming, and less widely available than immunofluorescence, we addressed the question of whether the FOAM technique can reliably identify the site of IgG deposits at the EBMZ, and distinguish BP from EBA. For this purpose, the technique was applied to perilesional skin from seven patients with BP and six with EBA, using computer-aided imaging of red-stained type VII collagen and green-stained IgG, according to previous findings. RESULTS: Digitized multicolor FOAM images of perilesional skin from patients with BP showed nonoverlap band patterns of green-stained lamina lucida IgG deposits (ultrastructurally proven) and red-stained type VII collagen. By contrast, FOAM images of EBA skin typically showed overlap patterns of green-stained sublamina densa IgG deposits and red-stained type VII collagen. These findings were observed also in skin tissue stored in Michel's transport medium or stored frozen for 15 years. CONCLUSIONS: The computer-aided FOAM technique may have great potential in distinguishing between IgG deposits above (BP) and just below (EBA) the lamina densa of the EBMZ in skin tissue. The technique is not as simple as saline-split skin methodology but offers more flexibility, and it certainly is quicker and less expensive than electron microscopy. Furthermore, the use of digitized fluorescence images offers improved possibilities for evaluating the various "linear" patterns of immune reactant deposition at the EBMZ in subepidermal bullous autoimmune skin diseases.