Objective: Nowadays, necrotizing periodontal diseases have a low prevalence; however, a better understanding of the etiopathogenesis of these diseases is necessary for determining more adequate preventive and therapeutic strategies. Method and Materials: From a pool of 1,232 HIV-infected patients, 15 presented with necrotizing periodontal diseases, which were evaluated by full-mouth periodontal clinical measurements. Subgingival biofilm samples were collected from necrotizing lesions of six of these individuals. The presence and levels of 47 bacterial species were determined by checkerboard DNA-DNA hybridization. Results: All 15 patients (10 had severe immunodeficiency) had been infected sexually. Thirteen patients were taking antiretroviral medication (66.7% undergoing highly active antiretroviral therapy). Regarding necrotizing periodontal diseases, necrotizing ulcerative gingivitis (60%) was more prevalent than necrotizing ulcerative periodontitis (40%). The frequency of supragingival biofilm and bleeding on probing ranged from 11.5% to 59.2% and 3.0% to 54.0%, respectively, whereas the mean probing depth and clinical attachment level were between 1.48 and 2.61 mm and 1.30 and 2.62 mm, respectively. Species detected in high prevalence and/or counts in necrotizing lesions included Treponema denticola, Eikenella corrodens, Dialister pneumosintes, Enterococcus faecalis, Streptococcus intermedius, Aggregatibacter actinomycetemcomitans, and Campylobacter rectus. In contrast, Parvimonas micra, Prevotella melaninogenica, Fusobacterium nucleatum, Eubacterium nodatum, and Helicobacter pylori were observed in the lowest mean prevalence and/or counts. Conclusion: Necrotizing periodontal disease lesions in HIV-infected patients present a microbiota with high prevalence and/or counts of classical periodontal pathogens, in particular T denticola, as well as species not commonly considered as periodontal pathogens, such as E faecalis and D pneumosintes. In addition, these individuals with necrotizing periodontal disease frequently display severe immunodeficiency and AIDS-defining diseases such as tuberculosis. Keywords: HIV, microbiology, necrotizing ulcerative gingivitis, periodontitis