Intravascular contrast agents remain a common cause of acute renal dysfunction, and prolonged intravenous saline administration prior to contrast exposure has been recommended in patients with elevated creatinine levels. For patient convenience and to minimize cost, same-day procedures have become common in patients with moderate renal insufficiency, despite little evidence to support this practice. A randomized trial was therefore performed to compare overnight hydration with bolus hydration in patients with moderate renal insufficiency. Although no overall change in creatinine was seen at 24 or 48 hours, four of the 37 patients (10.8%) in the bolus group versus none of the 26 in the overnight hydration group developed contrast associated nephropathy (CAN; p = 0.136). These results should at least question whether bolus hydration should be considered an equivalent alternative to overnight hydration in patients at risk for CAN undergoing angiography, and encourage further evaluation of this issue.