Almost all extensive tumors of the head or neck generate a fetor very unpleasant for the patient, his family, and the nursing staff. Thirty-one patients suspected of having a microbial superinfection of the tumor masses caused by anaerobes or an aerobe-anaerobe mixed infection were examined, and tumor samples from the oral cavity, throat, and larynx were obtained. Anaerobes could be demonstrated in every case, especially Bacteroides melaninogenicus. In the antibiotic treatment, clindamycin (Cleocin) proved to be superior to metronidazole (Clont). Within 12 to 24 hours after initiation of treatment, at the most 36 hours, the fetor almost always disappeared. The odor was stopped as long as antibiotic therapy continued.