The objective is to describe a case of probable aminoglycoside-induced Fanconi syndrome and make clinicians aware of the existence of this underrecognized and underdiagnosed complication in patients treated with a prolonged course of high-dose aminoglycosides. A 53-year-old man admitted for recurrent infective exacerbations of chronic bronchiectasis already colonized with Pseudomonas aeruginosa was treated intermittently with intravenous gentamicin (320 to 560 mg/d) for a total of 4 months to a total cumulative dose of 9.4 g. The patient developed profound hypophosphatemia, hypocalcemia, hyperphosphaturia, and aminoaciduria. Electrolyte disturbances persisted until gentamicin therapy was stopped, recurred with rechallenge, and did not correct with calcium and phosphate supplementation. This case shows that prolonged exposure to high-dose aminoglycoside therapy can be associated with Fanconi syndrome, which is a manifestation of proximal tubular dysfunction. There are only a few case reports to date of Fanconi syndrome as a probable complication of high-dose aminoglycoside therapy. The Naranjo Adverse Drug Reaction probability scale score indicated that this was a probable adverse reaction associated with administration of high-dose aminoglycosides. The differential diagnosis of electrolyte disturbances as a manifestation of proximal tubule dysfunction and type 2 renal tubular acidosis is vast; however, Fanconi syndrome needs to be considered in patients treated with high doses of aminoglycosides for longer than 6 days, after more common causes of hypophosphatemia are excluded.