Empyema due to Streptococcus mutans occurred following dental manipulation in two patients with periodontal disease. Isolation of this dental pathogen from pleural fluid localized the site of origin of the empyemas to the oropharynx and precluded the need to search for a remote intra-abdominal source of these pleuropulmonary infections. Although there is controversy regarding the susceptibility of S mutans to penicillin, the isolates from our patients were markedly sensitive to the drug, and both patients were cured with penicillin therapy and surgical drainage.