PURPOSE: To analyze ultrasonographic (US) features of focal xanthogranulomatous pyelonephritis (XGP). MATERIALS AND METHODS: US features of 15 patients with pathologically proven focal XGP were retrospectively analyzed by two radiologists who reached a consensus, in terms of the location, margin, size, and echo texture of the mass-like lesion, and presence or absence of associated calculi, lymphadenopathy, or local extension. These US findings were compared with CT findings. The patients were checked for the clinical symptom and sign through the review of hospital records. RESULTS: Thirteen of 15 patients were adults and 2 were children, and 10 were males. Their age ranged from 6 to 57 (mean, 39) years at presentation. Eight of 15 cases (53%) were in right kidney, and on US scan 12 cases (80%) were well circumscribed. The size of the mass ranged from 2.5 to 5.8 (mean, 3.8) cm. Of 15 masses, 13 (87%) were solid and two were cystic. Thirteen solid masses were composed of 10 masses (77%) with inner hypo- or anechoic foci and the other three masses without inner hypo- or anechoic foci, and the preoperative diagnosis was either renal cell carcinoma (n=11) or Wilms' tumor (n=2). The preoperative diagnosis in two cystic lesions (13%) was renal abscess. Renal calculi were found in one case, but lymphadenopathy or local extension was not depicted. Clinical symptoms and signs of flank pain, fever, leukocytosis, or anemia were found in 11 of 15 patients. CONCLUSION: Focal XGP revealed US features of solid or cystic masses confined within the renal outline mimicking renal tumor or abscess. US features, however, in association with clinical findings and other imaging findings (such as CT or MR imaging) may help the differential diagnosis of this lesion.