['There is limited data regarding the effects of preoperative dehydration on postoperative renal function. We sought to identify associations between hydration status before operation and postoperative acute renal failure (ARF) in patients undergoing colorectal resection.', 'The NSQIP database was used to examine the data of patients undergoing colorectal resection from 2005 to 2011. We used preoperative blood urea nitrogen (BUN)/creatinine ratio >20 as a marker of relative dehydration. Multivariate analysis using logistic regression was performed to quantify the association of BUN/Cr ratio with ARF.', 'We sampled 27,860 patients who underwent colorectal resection. Patients with dehydration had higher risk of ARF compared to patients with BUN/Cr <10 (AOR, 1.23; P\u2009=\u20090.04). Dehydration was associated with an increase in mortality of the affected patients (AOR, 2.19; P\u2009<\u20090.01). Postoperative complication of myocardial infarction (MI) (AOR, 1.46; P\u2009<\u20090.01) and cardiac arrest (AOR, 1.39; P\u2009<\u20090.01) was higher in dehydrated patients. Open colorectal procedures (AOR, 2.67; P\u2009=\u20090.01) and total colectomy procedure (AOR, 1.62; P\u2009<\u20090.01) had associations with ARF.'] Does preoperative dehydration increase risk of postoperative acute renal failure in colon and rectal surgery?