We describe a rare case of a malignant tumour of the rete testis which was diagnosed incidentally during scrotal surgery for an apparently benign left hydrocele. Adenocarcinoma of the rete testis is a malignancy with a poor prognosis. The beneficial role of prophylactic retroperitoneal lymphadenectomy is controversial due to the extremely low number of reported cases and the occasional description of primary lymphatic spread outside the retroperitoneal space.