Background/Aims: In esophageal cancer patients, postoperative pneumonia frequently occurs. In the oral cavity, dental plaque is the major reservoir of bacteria, and it is possible that oral bacteria are aspirated into the upper respiratory tract after an esophagectomy. We evaluated the interaction between preoperative dental plaque and postoperative pneumonia in patients undergoing esophagectomy. Patients and methods: Thirty-nine patients of thoracic esophageal cancer who underwent an esophagectomy were investigated. Preoperatively, dental plaque was collected and the bacterial flora investigated. If postoperative pneumonia occurred, the sputum was harvested and the pathogens were evaluated. Result: Postoperative pneumonia was observed in 14 patients (35.9%): 5 (71.4%) of the 7 patients in the pathogen positive group developed postoperative pneumonia, and 9 (28.1%) of the 32 patients in pathogen negative group developed. In 2 (40.0%) of 5 patients who had pathogenic bacteria in the preoperative dental plaque, the same pathogenic bacteria were also identified in the postoperative sputum. Conclusion: Pathogens in preoperative dental plaque are risk factors for postoperative pneumonia following thoracotomy in patients with thoracic esophageal cancer.