A 10-year-old boy presented with a 2.5-week history of right leg pain and limp. A right flank mass was noted by a parent on the day of presentation. The child's past medical history was remarkable for perforated appendicitis treated with an interval laparoscopic appendectomy 2 years before this presentation. Abdominal and pelvic computed tomography revealed a retroperitoneal mass with calcifications, suggestive of a retained appendicolith with abscess formation. This case illustrates the importance of considering very late complications of appendicitis in patients presenting with fever and abdominal or flank pain or masses. © 2010 by Lippincott Williams & Wilkins.