Necrotizing ulcerative gingivitis is an acute progressive and very painful infection associated with Prevotella intermedia, fusiform bacteria and spirochetes. It occurs mostly in combination with emotional stress, smoking, poor diet, poor hygiene and HIV infection or other diseases with immunosuppression. The papillae appear bright red, swollen and ulcerative. Gingival spontaneous bleeding and pseudomembrane formation often coincide with bad breath and necrosis of the gingival margin or/and the tips of the interdental papillae. Therapy includes debridement of the necrotic tissues, improvement of the oral hygiene, smoke abandonment, oral rinses, analgesics and appropriate antibiotics.