IRC are rare entities characterized by fever and unilateral flank pain resistant to antibiotic therapy. In our series of nine IRC in eight patients, the predominant sonographic finding was an abnormal echogenic texture of the cyst contents, in contrast to previous reports that stressed the frequency of wall thickening as a sign of infection. Sonographically guided cyst aspiration and drainage of eight cysts in seven patients provided diagnostic material and resulted in relief of symptoms. Although the cause of IRC is uncertain, these entities may be more common than previously suspected.