ABSTRACTBACKGROUND: Diabetes insipidus renalis has only occasionally been reported in ifosfamide-induced renal Fanconis syndrome, but in two studies on subclinical renal impairment, low morning urine osmolarity was found in high frequencies. This study was performed to assess the frequency of defective concentrating capacity in patients with ifosfamide-induced renal Fanconi’s syndrome or severe impairment of proximal tubular function. PATIENTS AND METHODS: Seven patients with overt Fanconi’s syndrome and 5 with a generalized but subclinical tubulopathy were examined. Beside proximal tubular solute reabsorption and estimation of glomerular filtration rate, urinary osmolarity was measured after overnight fast and DDAVP (1-D-amino-8-D-arginine vasopressin) testing. RESULTS: Five out of 7 patients with overt Fanconi’s syndrome, but no patient with only subclinical tubular damage, had decreased osmolarities by at least one test. Increased sodium excretion was additionally found in 4 of these patients. CONCLUSION: Impaired renal concentrating capacity is a rare event in ifosfamide-induced renal dysfunction and confined to patients with overt Fanconi’s syndrome. These patients should, however, undergo evaluation of concentrating capacity and renal sodium handling as decreased concentrating capacity and increased sodium excretion would render a patient at risk for dehydration episodes.