Intestinal aerobic Gram negative bacilli (IAGB) are frequently associated with wound infection after head and neck surgery. The origin of these organisms has not been previously identified. The stomach was investigated as a potential endogenous source. Saliva was examined in 15 patients 24 h before surgery. Following surgery, saliva and gastric aspirates were cultured daily for 10 days. All feeds were sterile. Though IAGB were never isolated pre-operatively, 29 species were detected in both the mouth and stomach of 14 patients post-operatively. Biochemically identical strains, initially present in the stomach, were detected 1-4 days later in the mouth in 8 patients. Three patients developed fistulae associated with IAGB but frank wound infection did not occur. Evidence demonstrating the gastric origin of faecal micro-organisms in the operative field and upper aerodigestive tract is presented and discussed.