A renal carbuncle (cortical abscess) is an important and treatable consideration in the differential diagnosis of renal mass lesions. Diagnosis is accomplished using a combination of clinical, urographic, arteriographic and ultrasonographic findings. Ultrasound is an accurate and less invasive alternative to arteriography. Needle aspiration of the abscess contents may give a bacteriologic diagnosis. Non-surgical treatment with antibiotics alone may be curative. Five cases are presented, and clinical and radiologic features are reviewed.