2 intraoral lesions associated with human immunodeficiency virus (HIV) infection have recently been described: an atypical gingivitis and a rapidly progressive periodontitis. The microbiota associated with these gingival and periodontal lesions was investigated. Subgingival plaque samples were taken from 45 HIV-seropositive homosexual men and from 44 HIV-seronegative control subjects. Each sampled site was clinically and radiographically classified as HIV-associated gingivitis, HIV-associated periodontitis, healthy in an HIV-seropositive subject, or healthy, conventional gingivitis or classical periodontitis in a control subject. Plaque samples were examined by indirect immunofluorescence with polyclonal antisera to detect Bacteroides gingivalis, B. intermedius, Fusobacterium nucleatum, and Actinobacillus actinomycetemcomitans. Anaerobic culturing was used to detect black-pigmented Bacteroides species, Fusobacterium species, and A. actinomycetemcomitans to confirm the immunofluorescence findings. We detected B. gingivalis, B. intermedius, F. nucleatum, and A. actinomycetemcomitans in significantly more HIV-periodontitis sites (80, 65, 59 and 61% of sites, respectively) and HIV-gingivitis sites (61, 70, 52 and 52%, respectively) than in HIV-seropositive healthy and control sites (p less than 0.05). The results indicate that the microbiota found in HIV-periodontitis is similar to that of classical periodontitis. In contrast, however, the microbiota associated with HIV-gingivitis is strikingly different from that of conventional gingivitis. The similarity in the prevalence of periodontopathic organisms in both HIV-gingivitis and HIV-periodontitis suggests that the HIV-gingivitis lesion may be a precursor to the tissue destruction observed in HIV-periodontitis. Hence, early detection and treatment of the HIV-gingivitis lesion may prevent the rapid and extensive breakdown of periodontal tissues associated with HIV-periodontitis.