In patients with no known systemic disease or immune dysfunction, necrotizing periodontitis (NUP) appears to share many of the clinical and etiologic characteristics of necrotizing ulcerative gingivitis (NUG) except that patients with NUP demonstrate loss of clinical attachment and alveolar bone at affected sites. In these patients, NUP may be a sequela of a single or multiple episodes of NUG or may be the result of the occurrence of necrotizing disease at a previously periodontitis-affected site. The existence of immune dysfunction may predispose patients to NUG and NUP, especially when associated with an infection of microorganisms frequently associated with periodontal disease such as Treponema and Selenomonas species, Fuscobacterium nucleatum, Prevotella intermedia, and Porphyromonas gingivalis. The role of immune dysfunction is exemplified by the occasionally aggressive nature of necrotic forms of periodontal disease seen in patients with HIV infection or malnutrition, both of which may impact host defenses. Clinical studies of HIV-infected patients have shown that patients with NUP are 20.8 times more likely to have CD4+ cell counts below 200 cells/mm3. However, these same studies have demonstrated that most patients with CD4+ cell counts below 200 cells/mm do not have NUP, suggesting that other factors, in addition to immunocompromisation, are involved. Further studies are needed to define the complex interactions between the microbial, or viral, etiology of necrotic lesions and the immunocompromised host. It is, therefore, recommended that NUG and NUP be classified together under the grouping of necrotizing periodontal diseases based on their clinical characteristics.