After presenting the clinical history of two young patients having retroperitoneal neoplasms of the seminoma type, we have discussed the general problem of partial or complete spontaneous regression of malignant tumours. On the basis of clinical and histological elements, and following a compilation of the literature, we have arrived at the conclusion that these neoplasms were in all probability retroperitoneal metastases of testicular seminomas having undergone complete spontaneous involution.