The role of glutamine in metabolic processes has been quite extensively researched and described, though no consensus exists as to its role in treatment. It is considered significant mainly for ICU patients; though no clear criteria for including glutamine in treatment have been defined. It is known that glutamine should be administered as a complement to nutritional treatment, and not independently. The purpose of the paper was to identify practical criteria for determining the clinical benefits of glutamine supplementation. The study was performed in the years 2007-2015 at the 1st Department of General and Transplantation Surgery and Nutritional Therapy of the Lublin Medical University in Lublin, Poland. It included patients scheduled for surgery due to a gastrointestinal cancer. The final study group included 105 patients, 48 female and 57 male. We found that low blood concentration of glutamine was correlated with a higher incidence of postoperative complications. ROC analysis allowed for identification of glutamine concentration below which there is a very high risk of complications. The threshold glutamine value identified was 205.15 nmol/ml. Low total lymphocyte count and serum albumin concentration can help identify patients in whom glutamine supplementation can decrease postoperative complication incidence, especially in the case of malnourished patients. Glutamine supplementation before a scheduled surgical procedure may benefit patients with a preoperative glutamine concentration below 205.15 nmol/ml. Glutamine supplementation can benefit malnourished patients.