The effect of immunonutrition vs standard polymeric nutrition in the preoperative phase on patient outcomes is still unclear, which could be due to contributing factors such as poorly reported compliance and unequal provision of protein, energy, and volume. The aim of this study was to examine the effects of preoperative immunonutrition compared with standard polymeric supplements matched for energy, protein, volume, length of stay, and postoperative complications in pelvic exenteration surgery, taking into consideration compliance. This was a single-blinded, randomized, controlled trial in patients undergoing pelvic exenteration surgery. Fifty-two participants were randomly assigned to consume 3 immunonutrition supplements/d for 5 days preoperatively, and 56 participants were randomly assigned to consume 3 standard polymeric supplements/d for 5 days preoperatively. Primary and secondary outcome measures were assessed. Compliance with each nutrition regime was recorded during the intervention, and subanalysis was conducted. There was no significant difference between immunonutrition and standard polymeric supplements on length of stay (P = 0.988) or postoperative complications (P = 0.179) after pelvic exenteration surgery. Compliance with nutrition supplements was significantly less in malnourished (n = 33) compared with well-nourished participants (n = 74) (P = 0.016). Compliance varied between study groups, although not significant. Preoperative immunonutrition did not significantly impact length of stay or postoperative complications compared with standard polymeric supplements after pelvic exenteration surgery, despite controlling for protein, energy, and volume. Compliance, protein, and energy may considerably impact the effect of immunonutrition, particularly in malnourished patients.