We present a case of focal medullary sponge kidney (MSK) that mimicked a renal tumor. Evaluation of a patient with history of macrohematuria revealed a left renal mass of 3-cm diameter. T 1 -weighted magnetic resonance (MR) images revealed a mass of mixed intensity protruding toward the renal sinus. On fat-saturated T 2 -weighted MR images, the lesion's remarkable hyperintensity suggested the presence of an aggregation of tiny cysts. On diŠusion-weighted MR images, the mass also demonstrated

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... h intensity, and its apparent diŠusion coe‹cient was partly decreased (1.12×10 -3 mm 2 /s). On computed tomography, precontrast images revealed no calciˆcation in the mass. Although slight enhancement was seen in the corticomedullary phase, thick and dense streaks of contrast radiating peripherally were identiˆed in the mass in the excretory phase. Focal MSK was diagnosed. We discuss the potential of MR imaging for diagnosing focal MSK.