We retrospectively studied the bacteriology and clinical features of empyema thoracis and lung abscess caused by viridans streptococci in 72 patients seen from January 1984 to September 1996. A total of 76 strains of viridans streptococci were isolated, of which the most common isolates were Streptococcus constellatus (21 strains), S. intermedius (17), and S. sanguis (10). Species belonging to the S. milleri group accounted for the majority (68%) of isolates. In 38 (53%) patients these organisms were recognized as the sole pathogens. Of the 72 patients, 53 had empyema, 14 had lung abscesses, and five had both empyema and lung abscess. Forty-six (64%) patients had underlying diseases. Of these, malignancies were the most common (17 patients), followed by diabetes mellitus (12 patients) and central nervous system diseases (10 patients). Of the 48 patients who underwent chest-tube drainage, 27 (56%) received further treatments, including intrapleural streptokinase (18 cases), surgery (9), and both intrapleural streptokinase and surgery (3). Two (14%) of the patients with lung abscess alone underwent surgical treatment. Although all viridans streptococcal isolates were susceptible to penicillin, the patients in the study had a high mortality (21%). Univariate and multivariate analysis of data for patients with empyema alone (n = 53) showed a significantly increased risk of death in those with underlying malignancy (OR = 16.0, p = 0.023) and those with non-S. milleri-group isolates (OR = 3.72, p = 0.030). These data imply a strong clinical significance of viridans streptococci in the pathogenesis of empyema and lung abscess, as well as the need for species identification of viridans streptococci in patients with pleuropulmonary diseases.