Castleman's disease usually manifests as a solitary mediastinal tumor and only rarely as an isolated retroperitoneal mass. This disorder is often undiagnosed or misdiagnosed. Thus, only very few patients have been reported and little information is available in the literature. The definitive diagnosis is based on postoperative pathological findings. We report a case of a 57-year-old female with a Castleman's tumor located superomedial to the upper pole of the right kidney that mimicked an adrenal neoplasm. The mass was surgically resected, and the histopathological diagnosis of the resected tissue was hyaline-vascular type of Castleman's disease. Although retroperitoneal Castleman's disease is rare, it should be considered in the differential diagnosis of retroperitoneal masses.