We report on a case of solitary Castleman's disease that had an unusual presentation. A 29-year-old Filipino woman who had a history of intermittent right lower abdominal pain for several years was admitted to the Princess Margaret Hospital because of a sudden exacerbation of the abdominal pain. Ultrasonography had previously detected a multilocular right ovarian cyst of approximately 6.5 cm in diameter. Intra-operative findings, however, revealed a retroperitoneal presacral cystic tumour of approximately 7 cm in diameter, which was unrelated to the ovaries. The tumour was removed and found to be well defined and measure 7 x 5 x 4 cm. The cut surface revealed homogenous light-brown tissue at the periphery. The central part showed cystic spaces of 1- to 3-cm diameter. Histological examination of the tumour led to the diagnosis of hyaline vascular-type Castleman's disease. Cystic spaces were visible within the infarcted tissue; extensive cystic changes in Castleman's disease are unusual and may have caused the diagnostic difficulty.