Although parenteral nutrition is a vital method of delivery essential nutrients in patients with malnutrition associated to gastro-intestinal insufficiency, its inappropriate use can increase the risk of complications and incur unnecessary expenses. Our goal was to evaluate the influence of both, the presence of the Nutritional Support Unit and the implementing clinical practice guidelines on post-operative nutritional status, complications and length of stay among patients undergoing elective colorectal cancer surgery. Prospective and observational study: Three period times were included-the year during the guidelines elaboration (A), and the first (B) and the second year (C) after their implementation. All patients submitted to elective colorectal cancer surgery at least 18 years of age were included (A: n=297; B: n=103, and C: n=149). WE ANALYSED: Nutritional status (NS) on admission to hospital and at discharge, use of post-operative parenteral nutrition (PPN), incidence of post-operative complications (PC), number of days of nil by mouth following surgery (NPO), and hospital length of stay (LOS). Although the prevalence of malnutrition on admission was low, an increment was observed during the hospitalisation time. However, only in the first period time, the increment was significantly different (A: from 8.4% to 19.5%, p<0.001; B: from 3.9% to 8.7%, and C: from 4.7% to 6.7%). Globally, the use of PPN decreased (A: 79.1%, B: 47.0%, and C: 12.8%; p<0.001). This behaviour was mainly observed in well-nourished patients (use of PPN in well nourished, A: 79.3%, B: 44.4%, and C: 11.3%; p<0.001). Significant differences were observed in the global incidence of PC (A: 27.9%, B: 28.2%, and C: 8.1%, p<0.001). Furthermore, PC was higher in well-nourished patients with PPN versus without PPN, with significant differences in B and C periods (A: 29.3% vs. 25.0%; B: 38.6% vs. 18.8%, p=0.004; C: 31.3% vs. 4.8%, p=0.003). The NPO was higher in patients without PPN in period A (7 d vs. 5 d, p<0.001) and higher in those with PPN in period C (8 d vs. 6 d, p=0.035). All in all, LOS decreased significantly during the study period time (A: 16 d, B: 13 d, and C: 11 d, p<0.001). The presence of Nutritional Support Unit and clinical practice guidelines for colorectal cancer management and treatment, optimised the use of hospital resources, namely unnecessary use of parenteral nutrition, reduction along with decrease in number of complications and length of hospital stay.