Objectives:To assess the value of using intramucosal pH (pHi) measurements to evaluate the viability of the gastric tube after thoracic esophagectomy, and to determine whether these measurements may be used for early prediction of anastomotic insufficiency.Design:Prospective, observational study.Setting:University hospital in Japan.Patients:Thirty-nine patients who had undergone thoracic esophagectomy as a treatment for esophageal cancer.Interventions:The blood flow within the gastric tube was measured using a laser Doppler flowmeter during surgery. Periodic measurement of the pHi within the gastric tube (gastric pHi) began during surgery and continued until the second postoperative day. In 30 patients, the pHi within the rectum (rectal pHi) was measured simultaneously with the gastric pHi. The patients were divided into two groups: those patients who experienced anastomotic insufficiency constituted the leakage(+) group (n = 13); those patients who did not experience these complications were designated the leakage(−) group (n = 26).Measurements and Main Results:The gastric pHi values correlated significantly with simultaneous measurements of the blood flow at the anastomotic site (p< .01). The postoperative gastric pHi values increased gradually in the leakage(−) group but stopped increasing after surgery in the leakage(+) group. The rectal pHi values increased gradually after surgery in both groups. Furthermore, there was a significant difference between the two groups when their gastric pHi values were subtracted from their rectal pHi values from the morning of the first postoperative day until the morning of the second postoperative day (p< .05).Conclusions:The gastric pHi values well reflected the viability of the gastric tube, especially when combined with the rectal pHi values. By measuring pHi, we can more accurately predict the risk of anastomotic insufficiency earlier after surgery and therefore give those patients who need it additional care to improve the viability of the gastric tube.