[Retroperitoneal mucinous cystadenoma]

The authors report an unusual case of retroperitoneal mucinous cystadenoma in contact with the isthmus of a horseshoe kidney and associated with congenital absence of the left ovary. The retroperitoneal site is very unusual, as only about twenty cases have been reported in this site. The pathogenesis of these tumours remains unclear and the authors discuss the various hypotheses proposed to date.