We reviewed the clinicopathological characteristics of lung abscesses retrospectively. We analyzed 89 patients hospitalized from July 1984 to May 2009. Most were men (76/89). There were large proportions with alcohol consumption (29.2%) and dental caries or gingivitis (60.7%). Furthermore, those without other diseases accounted for only 13.5%. Predominant infectious species were clear in 43 cases (48.3%) including identification of bacteria. The identification rate of predominant bacteria improved from 38.5% to 56.0% after initiation of the introduction of expectoration culture, bronchoscopic specimen collection and gingival culture in 2003, facilitating clarification of the predominant bacteria. The Streptococcus anginosus group with predominant bacteria being slightly aerobic streptococci, anaerobic bacterium, and aerobic bacterium was detected in 10, 12, and 31 cases, respectively. The improvement in the identification rate of predominant bacteria was achieved by carrying out examination with close liaison with the staff of our inspection room. In selecting antimicrobials based on diagnostic significance, we should focus on positive identification of predominant bacteria, a factor which appears to have major clinical significance.