This study aimed to clarify the effectiveness of post-operative oral care, including tooth brushing, denture cleaning and tongue cleaning, after digestive tract surgery. Subjects included 30 elderly patients aged 60-98 years (74.9 +/- 7.8 years) who underwent digestive tract surgery. Subjects were randomly divided into an intervention group and a control group. In the intervention group, the following oral care was provided daily over a 5-min period in the morning starting at baseline (day of surgery) and continued for 5 days: gargling with povidone iodine, tooth brushing, denture cleaning using a special brush and tongue cleaning using a tongue brush. In the control group, the subjects only gargled with povidone iodine. We compared the following variables between the two groups at baseline and 5 days later: the sensation of dry mouth, intra-oral gas concentration, pulmonary sounds, body temperature and bacterial flora. The number of patients with abnormal pulmonary sounds (dry or moist rales) increased from 1 to 2 in the intervention group and from 0 to 4 in the control group (P < 0.05). The average number of bacterial species per subject for the control group was 3.64 +/- 1.34 pre-operatively and 3.50 +/- 1.74 post-operatively, whereas that for the intervention group was 3.08 +/- 0.95 pre-operatively and 2.62 +/- 0.65 post-operatively. In the intervention group, there was a significant decrease in the number of bacterial species (P < 0.05). These findings indicate that post-operative oral care in elderly patients undergoing digestive tract surgery lowers the number of bacterial species found in the oral cavity. This effect, in turn, might improve respiratory function.