PURPOSE: To define the imaging patterns of xanthogranulomatous pyelonephritis (XGP). MATERIALS AND METHODS: The demographic, clinical, and imaging findings of 21 cases of pathologically proven XGP in 20 patients (bilateral in one) were evaluated. The findings of ultrasonography and CT were retrospec-tively evaluated with regard to distribution and extent of the disease, kidney size, the presence of calculi, hy-dronephrosis, and renal function. The findings were assessed by two radiologists, who established a consen-sus. Imaging and pathologic findings were compared. RESULTS: Sixteen of the 20 patients were female, and 19 were adults. Their age ranged from 3 to 61 (mean, 45) years. In all patients except one, the disease was unilateral (right: left=13:16). In one patient, XGP was bilater-al, and there were thus 21 cases. Seventeen (81%) of these were diffuse, and four (19%) were focal; extrarenal extension occurred in 13 cases (62%), among which ipsilateral pleural effusion was noted in two. The kidney was enlarged diffusely in 12 cases (57%), and focally in three (14%); urinary calculi were present in 16 cases (76%), with staghorn calculi in four of these; and hydronephrosis occurred in 17 (81%). Impairment of ipsilat-eral renal function was noted in 13 cases (62%). Clinical findings of inflammation such as fever, pyuria, bac-teriuria, or leucocytosis were noted in all patients. CONCLUSION: In addition to nephromegaly, renal function impairment, and urinary obstruction due to calculi, which are typical features of XGP, the condition may also show variable imaging findings. If the images ob-tained in the case of a middle-aged woman with clinical findings of urinary infection are atypical, we believe that XGP should be included in the differential diagnosis.