Castleman disease or angiofollicular lymph node hyperplasia is a rare disease of unknown cause with polymorphic clinical features that raises many diagnostic and therapeutic problems. The unifocal, localized, pseudoneoplastic form, first described in 1956, has a good prognosis, but the multifocal form has a more aggressive course. Three histological types have been identified: hyaline vascular, plasma cell and mixed. The authors present a case of hyaline vascular unifocal Castleman disease. A retroperitoneal mass was discovered incidentally during follow-up ultrasound examination in a 38-year-old woman treated for recurrent renal stones. On CT examination, a solid mass measuring 9 x 8 x 6 cm raised a problem of differential diagnosis with other retroperitoneal masses. CT-guided biopsy was inconclusive and the diagnosis was finally established by tumour excision and histological examination. The immediate and subsequent postoperative course was uneventful.