We report the clinical data, light and electron microscopic findings, therapy and clinical course of three cases treated for acute necrotizing ulcerative gingivitis. The following results were obtained. 1. An ulcerous lesion was observed in the gingiva of all three cases, with case I also showing a depressed lesion in the ulcerous region. 2. In all three cases, the gingival sections displayed ulceration and showed fibrin deposition and neutrophil infiltration on the surface of the ulcerous region. Congested blood vessels and neutrophil infiltration also were observed in the connective tissue beneath the ulcerous region. 3. Many microorganisms, fibrin, cell debris, and enlarged intercellular spaces of the epithelium were seen on the surface on the ulcerous region. 4. Localized treatment, mainly plaque control, was found to be effective clinically and histopathologically.