BACKGROUND: Surgery has become the treatment of choice for head and neck cancer (HNC) in most cases. Preoperative fasting abbreviation and nutrition screening have been suggested to reduce the occurrence of postoperative complications. This study aimed to evaluate the addition of whey protein in the preoperative fasting abbreviation and to analyze the association of nutrition status on postoperative complications in patients with HNC. METHODS: A randomized, single-blind clinical trial was performed. Patients recruited from March to November 2018 at a national cancer reference center in Brazil were divided into 2 groups: intervention group (clear fluids with carbohydrate plus whey protein [CHO-P]) and control group (clear fluids with carbohydrate only [CHO]). All patients were evaluated by the Patient-Generated Subjective Global Assessment (PG-SGA) and body mass index. Logistic regression analyses were performed to assess associations between the studied variables, generating odds ratios (ORs) and 95% confidence intervals (CIs). RESULTS: A total of 49 patients met the eligibility criteria and enrolled in the study. A PG-SGA score of 4-8, indicating moderate malnutrition (OR, 10.91; 95% CI, 1.05-112.91), was an independent factor that increased the risk of postoperative complication, whereas the CHO-P group (OR, 0.04; 95% CI, 0.01-0.26) was found to be an independent factor in reducing such risk. CONCLUSION: The addition of whey protein to clear fluids (CHO-P group) was associated with a reduced risk of postoperative complications compared with the CHO group. Furthermore, moderate malnourishment was associated with an increased risk of postoperative complications for patients with HNC.