Early postoperative hyperglycemia in non-diabetic patients is an important risk factor affecting postoperative complications and mortality. This study aimed at investigating the effects of early postoperative hyperglycemia on postoperative complications, hospital costs, and length of hospital stay in non-diabetic patients with gastrointestinal malignancies; data of 1,015 non-diabetic patients with gastrointestinal malignancies, who underwent surgical intervention between January 2010 and January 2012, were retrospectively evaluated. Records on fasting plasma glucose (FPG), liver function, and kidney function were collected before and one day after surgery. Correlation of early postoperative FPG levels with postoperative complications, hospital costs, and length of hospital stay was further assessed in non-diabetic patients with gastrointestinal malignancies. One day after surgery, FPG results were significantly increased compared to preoperative values. FPG levels greater than or equal to 9.13 mmol/L (or 164.34 mg/dL) were associated with significant increases in the incidence of postoperative complications, length of hospital stay, and hospital costs. An association is shown between FPG and postoperative hyperglycemia in non-diabetic patients undergoing surgery for gastrointestinal malignancies. Significant increases in postoperative complications among these patients suggest that measurement of early postoperative FPG levels is critical to identify patients with postoperative hyperglycemia.