Lung abscess is frequently caused by anaerobes that are difficult to diagnose by sputum examination. To evaluate diagnostic methods and bacteriology of lung abscesses, we retrospectively studied 44 consecutive lung abscess cases (37 men; 7 women; median age, 60 years) admitted and treated at our hospital from 2001 to 2010. The most frequent underlying disease was periodontitis (n=20, 45.5%). The diagnostic rate of causative pathogens by ultrasonography-guided fine-needle aspiration (n=2), computed tomography (CT)-guided fine-needle aspiration (n=19), sputum examination (n=37), and bronchoscopy (n=10) was 100, 68.4, 16.2, and 10%, respectively. In total, 43 causative pathogens were identified in 18 cases (40.9%), of which 12 (67%) had polymicrobial infections. Furthermore, anaerobes and bacterial species belonging to the Streptococcus anginosus group accounted for 55.8 and 14% of the 43 identified pathogens, and both were identified by examination of CT-guided fine-needle aspiration fluid in all cases, except for one patient. Every case was successfully treated with antibiotics. Anaerobes and species of the S. anginosus group are common causes of lung abscess, and CT-guided fine-needle aspiration is a useful diagnostic tool for identifying these causative agents.