Current evidence suggests that periodontal disease may be associated with the development of a lung abscess or an empyema. Some microbes, including the Streptococcus anginosus group and obligate anaerobic bacteria, are recognized as important pathogens. We analyzed periodontal disease bacteria by polymerase chain reaction (PCR) assays of transbronchial biopsy (TBB) specimens, bronchial lavage fluid (BLF), or pleural effusion obtained from 7 cases (lung abscess, 3; and empyema, 4). In 1 case, anaerobes were isolated from BLF culture, but periodontal pathogens were detected in TBB specimens or pleural effusion by PCR assays in 6 cases (Porphyromonas gingivalis, 3; Tannerella forsythensis, 1; Treponema denticola, 1; and Prevotella intermedia, 1). In 1 case T. denticola of unknown pathogenicity was detected in the TBB specimens. PCR assays targeting periodontal disease bacteria are potentially useful for identifying the causes of respiratory infectious diseases, which are difficult to diagnose by anaerobic culture.