Figure 1 . Unenhanced axial CT image shows a welldefi ned complex cystic mass within the left upper quadrant. The mass contains multiple smaller, peripheral, thin-walled cysts with variable but predominantly low attenuation within a fl uid matrix ( * ). History A 44-year-old woman with no signifi cant medical history presented with urinary urgency and left fl ank pain. The patient was originally from Armenia and had been living in the United States for the past 20 years. At physical

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... the patient was afebrile, and there was palpable fullness in the left upper quadrant of the abdomen and left fl ank with no tenderness. Initial laboratory test results were normal except for an elevated white blood cell count of 14 (normal range, 4-11) and microscopic hematuria. Unenhanced computed tomography (CT), which was performed for evaluation of urinary tract calculus disease, resulted in the incidental discovery of a large, multiloculated, cystic left renal mass. The patient underwent subsequent multimodality evaluation. Imaging Findings Initial helical CT of the abdomen and pelvis performed for evaluation of renal colic revealed a large, left-upper-quadrant, complex cystic mass that measured 17 × 12 × 11 cm (Fig 1) . The mass originated from the upper pole of the left kidney and was composed of a thin-walled cyst that contained numerous smaller cysts. The mass displaced the left kidney inferiorly and the spleen anteriorly. No renal calculi were identifi ed, although a calcifi cation that was inseparable from the midportion of the right ureter was seen. The patient underwent cystoscopy, ureteroscopy, and bilateral retrograde pyelography for further evaluation of the urinary tract and renal mass (Fig 2) . The mass caused inferomedial deviation of the left renal pelvis, pronounced infundibular splaying, and caliceal distortion. The right renal collecting system, both ureters, and the bladder were otherwise normal in appearance, with no evidence of obstruction or fi lling defect. Magnetic resonance (MR) imaging of the abdomen helped to further characterize the left renal mass (Fig 3) . Multiplanar T2-weighted images showed a large cystic mass with a low-signalintensity rim, a high-signal-intensity internal matrix, and multiple smaller internal cysts. The "daughter" cysts were hypointense relative to intracystic fl uid matrix on T1-weighted images and isointense relative to fl uid on T2-weighted images. On the basis of these fi ndings, the mass was believed to be a renal hydatid cyst. Pathologic Evaluation Further laboratory test results revealed substantially elevated serum Echinococcus antibody titers, a fi nding that supported the diagnosis of renal hydatid disease. The patient opted for surgical treatment, and an open left nephrectomy was performed. Gross pathologic examination revealed a 17 × 12 × 11-cm cystic mass located in the superior aspect of the left kidney (Fig 4) . The mass was well demarcated by an apparent pseudocapsule and contained multiple free-fl oating internal cysts