Nephrotoxicity manifested by renal insufficiency is a well-known consequence of aminoglycoside administration in hospitalized patients. We report two cases of aminoglycoside-induced nephrotoxicity as manifested by proximal tubular dysfunction (Fanconi's syndrome). In the two patients reported, the tubular dysfunction occurred at the extremes of renal function. In both patients, tubular function returned to normal when the antibiotic was discontinued. Because of the widespread use of these antibiotics in the critically ill patient, it may be prudent to recognize this disorder when managing fluids, electrolytes, and nutrition.