BACKGROUND Acute kidney injury is a relatively common complication of aminoglycoside therapy that affects 10-20% of patients receiving antibiotics of this class. Although the vast majority of patients do recover, the presence of certain risk factors may alter the clinical presentation and result in permanent renal damage. Thus, aminoglycoside-induced nephrotoxicity is a major concern and early diagnosis is critical. The aim of the present study was to characterize the early stages of aminoglycoside-induced nephrotoxicity using proton nuclear magnetic resonance ((1)H NMR) spectroscopy of urine. METHODS We studied 19 previously healthy patients who were hospitalized in our clinic due to bacterial infections. Combined antibiotic treatment with amikacin (1 g once daily) and a beta-lactam antibiotic was instituted in all patients. Urine and blood samples were collected before and after 5 days of aminoglycoside treatment. RESULTS (1)H NMR spectroscopic data showed increased amounts of alanine and lactic acid (+138 and +255% compared to baseline values, respectively) and decreased hippurate (-50%) after aminoglycoside treatment. In addition, fractional excretions of sodium, magnesium and calcium were significantly increased (+271, +295 and +60%, respectively). These findings indicate that aminoglycosides can affect nephron tubules through two unrelated mechanisms that produce a partial 'Fanconi-like syndrome' and a 'Bartter-like syndrome'. However, because none of the study participants developed renal failure, these alterations are probably reversible and should not be used as sensitive or specific indicators of impending renal insufficiency. CONCLUSIONS Our study findings confirm that aminoglycosides can induce both proximal and distal renal tubular dysfunction. However, it remains unclear whether the early functional changes detected by (1)H NMR spectroscopy in patients treated with aminoglycosides are of prognostic value.