BACKGROUND: Anaerobic bacteria constitute a major part of the normal flora of the human skin, mucous membrane and intestinal tract, and can cause various infections. The incidence of anaerobic infections may differ greatly, depending on each country or hospital. METHODS: We evaluated the recent trends of anaerobic bacteria isolated from clinical specimens at Severance Hospital from 1986 to 1995. Specimens were cultured using thioglycollate medium and phenylethanol blood agar (PEBA) for 2-3 days under anaerobic condition. Identification of organism was based on conventional or commercial kit systems. RESULTS: During this period, a total of 2,664 isolates of anaerobic bacteria were obtained from 2,251 clinical specimens. The average number of anaerobes per specimen was 1.2. The frequent sources of isolation were specimens from the abdomen, followed by soft tissue, and head and neck. B. fragilis (46.3%) was the most frequently isolated gram-negative bacilli, and P. magnus (37.6%) and C. perfringens (18.8%) were the most frequently isolated gram-positive anaerobes. Abdominal, soft tissue, and head and neck infections were frequent clinical conditions. Among the anaerobe-positive specimens, only 16.8% yielded anaerobe alone while the remaining 83.2% revealed mixed infection with aerobic bacteria. CONCLUSIONS: It was concluded that B. fragilis is the most common species among gram-negative bacilli, and that P. magnus is the most common among gram-positive cocci. As well, the anaerobes are frequently isolated from specimens of the abdomen, head and neck, and soft tissue; and anaerobic infections are commonly mixed with aerobic bacteria.