The oropharyngeal aerobic gram-negative bacillary carriage, uncommon in healthy people, is high in patients with head and neck cancer. A marked increase in oral Gram-negative enterobacteria and pseudomonads has particularly been shown during the period of radiation therapy. The aim of the study was to determine whether these bacteria could play a role in the causation of irradiation mucositis. The current study demonstrated that aerobic and facultatively anaerobic gram-negative rods and cocci and Gram-positive bacteria showed a marked increase in head and neck–irradiated subjects. There was a weak but non-significant correlation between the bacterial flora and oral mucositis.