The objective of this study was to increase our understanding of the importance of members of the Streptococcus milleri (SM) group as respiratory pathogens, by studying the epidemiological and clinical features of thoracic infections caused by this group and comparing the epidemiology and prognosis of empyema caused by SM with cases of pneumococcal aetiology. The clinical histories and microbiology reports were reviewed in 27 cases of thoracic infection caused by SM over a period of 8 yrs. Cases of pneumococcal empyema that occurred during the same period were also analysed. Diagnoses were made of cases of empyema, including six with pneumonia and one with pulmonary abscess, three cases of pneumonia and two of mediastinitis. In 17 cases, SM was the only pathogen isolated. There was a history of instrument or surgical procedures on the digestive or respiratory tract in 59%. Secondary bacteraemia was documented in three cases. The treatment administered, a combination of antibiotics and surgery, was successful in 22 of 27 (81%) of cases. All strains were susceptible to penicillin. When the characteristics of the empyemas caused by monomicrobial SM infection were compared with those of pneumococcal aetiology from the same period of study, significant differences were found with respect to age, origin of the infection and the need for surgery. In conclusion, thoracic infections caused by Streptococcus milleri are largely pleural. They are polymicrobial in one-third of cases, commonly acquired in hospital and, in most patients, associated with major surgery and/or surgical procedures of the respiratory or digestive tract. The empyema frequently requires thoracotomy for complete resolution.