Renal cyst infection-pyocystis-is a potentially life-threatening complication of autosomal-dominant polycystic kidney disease. Differentiation of pyocystis from pyelonephritis is important for antibiotic management. A 56-year-old woman with autosomal-dominant polycystic kidney disease and recurrent urinary tract infections was admitted to the hospital with suspicion of pyelonephritis. CT and static planar In-labeled WBC examinations failed to show a specific focus of infection. Abdominal imaging with In-labeled WBC SPECT/CT revealed abnormal leukocyte accumulation within a solitary right renal cyst. Precise SPECT/CT localization of the infected renal cyst is illustrated along with comparative CT images.