¹Department of Veterinary Surgery, Faculty of Veterinary Medicine, Tokyo University of Agriculture and Technology, 3-5-8 Saiwaicho, Fuchu, Tokyo 183-8509, JAPAN DOI : 10.9775/kvfd.2017.18665 A 7-year-old intact female Large Münsterländer developed abdominal distention. Computed tomography showed absence of the right kidney and ureter, marked hydronephrosis, and left ureter dilatation. Five years later, the patient was hospitalized as an emergency. Diagnostic imaging showed a >350-mm-diameter cyst communicating with the left kidney displacing the abdominal organs, another approximately 150-mmdiameter cyst in the right kidney position, and marked parenchymal thinning of the left kidney. At laparotomy, a left kidney nephrostomy was established; 8.230 mL of urine were drained with paracentesis. There was no ureter connecting the left kidney and bladder, but an enlarged, ureterocele-like, tubular organ connected the left kidney to a urine-containing cyst on the right side. A short, tubular organ connected the urine-containing cyst on the right side with a bladder-like organ, but it showed almost no urine accumulation. Urine cultures tested positive for Klebsiella pneumoniae. Postoperatively, the patient was discharged after four days. This is the first case about unilateral renal agenesis (URA) in an aged dog. Some dogs may have undiagnosed URA. When URA manifests after a dormant period, cases may be severe, with total loss of appetite, abdominal distension, elevated inflammatory markers, and cyst-like phenomena on abdominal ultrasonography. In such cases, URA must be considered in the differential diagnosis. Keywords : Unilateral renal agenesis (URA), Aged dog, Urine accumulation, Urinary tract infection