Biopsy specimens from patients with granuloma annulare were studied by conventional light and immunofluorescence microscopy. In six of 20 patients, IgM was present in blood vessels of involved skin, and C3 was present in blood vessels in ten of 20 patients. In addition, IgM, C3, or fibrinogen were observed at the dermal-epidermal junction of eight patients. Necrobiotic areas contained fibrinogen. A retrospective histopathologic study of 38 biopsy specimens from other patients with granuloma annulare showed blood vessels with vessel wall necrosis, fibrinoid change, thickening, or occlusion in most specimens. Nuclear dust and extravasation of RBCs were also noted. These findings suggest that an immunoglobulin-mediated vasculitis may be involved in the pathogenesis of granuloma annulare.