Nine cases of granuloma annulare at different stages of development were investigated electron microscopically. Special attention was given to collagen changes, to the cellular infiltrate, and to vascular changes. The necrobiotic changes of the collagen were: loosening of the fibers at the borders of the bundles; swelling and loss of cross-striation of the fibrils; and complete degradation of the collagen, with precipitation of mucinous and fibrin material and of glycogen. In 2 cases "cross-banded filamentous aggregations" were present. In later stages new collagen synthesis occurred. In the infiltrate, histiocytes/macrophages were seen developing into epithelioid and giant cells. In addition, numerous active lymphocytes and, at later stages, some neutrophils were found. Vascular changes consisted of endothelial swelling and duplications of the basal lamina. A perivascular infiltrate was present, but no distinct vasculitis was seen. Our results indicate that the necrobiosis of the collagen without primary vasculitis may be an initial event in granuloma annulare. A delayed hypersensitivity mechanism is discussed with regard to pathogenesis.