In the past, vancomycin has been reported to cause renal failure during intravenous administration; however, more recently, such renal toxicity is alleged not to occur because of increased purity of the vancomycin preparations. In this study, 23 patients were prospectively examined during intravenous vancomycin administration for changes in renal function. Vancomycin was administered for an average of 15 days. The blood urea nitrogen (BUN) changes averaged +1.7 mg/dl and the creatinine changes averaged +0.06 mg/dl. Since the accuracy of the serum creatinine determination was +/- 0.3 mg/dl, clinically significant deterioration of renal function occurred in 4 patients or 17%. Even among these 4 patients with documented worsening of renal function, we suspect that deterioration was related to the infection being treated. With close monitoring of dosing, the propensity of vancomycin to cause nephrotoxicity may be less than once thought.