Ten adult patients with severe Bacteroides infections were treated with 0.9 approximately 1.8 g/day of parenteral or oral clindamycin, and a child was treated with 0.3 g/day orally. Remarkable responses and cures were obtained in all the patients, who had no underlying diseases and pure anaerobic infections; a case of sepsis, two cases of liver abscess, a case of subcutaneous abscess and a case of spinal epidural abscess. The other six patients who had ultimately fatal underlying diseases or mixed infections did not respond well to the combination of clindamycin and the other antibiotics therapy, althought bacteriological cures were obtained in all but two cases. Clindamycin was well tolerated and generally nontoxic, nevertheless it was administrated for long term (34 approximately 49 days). But transient development of transaminase was seen in a patient. The data suggested that clindamycin should be considered a first choice antibiotic for the treatment of an aerobic, especially, Bacteroides infections.