A 16-year-old boy presented with growth retardation and iron deficiency anemia. The disease was identified incidentally in the pararenal retroperitoneum after computed tomography and magnetic resonance imaging scans. A retroperitoneal lesion was removed in its entirety and was histologically confirmed to be a symptom of Castleman's disease of the unicentric plasma cell type. The unicentric plasma cell type appears so rarely in the retroperitoneum that a similar case has been reported only once. The patient was discharged on day 9 after surgery without significant complications and grew 18 cm within a year.