The case is a 38-year-old man in whom a solitary subcapsular left renal cortical mass was successfully resected. Comorbidities included a benign epididymal cyst and a history of nephrolithiasis. Computed tomographic imaging demonstrated a 1.8-cm enhancing mass in the anterior midregion of the kidney. An open partial nephrectomy was performed, and histopathologic examination established a diagnosis of the hyaline-vascular type of Castleman disease (CD). The patient had an uneventful postoperative course and has experienced no local or metastatic recurrence in the 10 months since surgery. CD localized in the kidney is an exceptionally rare occurrence but should be included in the complete differential diagnosis of solitary renal cortical mass lesions.