Although ultrasonography is regarded as the gold standard in the diagnosis of obstructive nephropathy, dilatation is sometimes not observed by ultrasonography. We report upon a case of minimally dilated obstructive nephropathy due to an ureter stone in a kidney donor with volume depletion. A 54-year-old man was admitted due to anuria and abdominal pain of 2 days duration. Ten years previously, his right kidney was donated for transplantation, and one month before admission, he abstained from all food except water and salt, for 30 days for religious reasons. He had lost 8 kg of body weight. On admission, he had clinical signs of volume depletion, i.e., a dehydrated tongue and decreased skin turgor. Laboratory data confirmed severe renal failure, his blood urea nitrogen level was 107.3 mg/dL, and his serum creatinine 16.5 mg/dL. The plain X-ray was unremarkable and ultrasonography showed only minimal dilatation of the renal collecting system. On follow-up ultrasonography, performed on the 5th hospital day, the dilatation of the collecting system had slightly progressed and a small stone was found at ureter orifice by cystoscopy. Removal of stone initiated dramatic diuresis with a rapid return of renal function to normal by the third day.