Five case histories involving extragonadal type-I Dixon-Moore seminomas are described, each having a large retroperitoneal tumor growth with, variously, lymph node or lung metastases, growth into or around organs, blood vessels and muscles, etc. The large tumor masses could be completely resected in some cases only. In three cases, post-operative combined radio- and chemotherapy have left the patients tumor-free for several years; in two cases radiotherapy given along resulted in a full remission. In one case, the undescended testis, in another, the fibrous tissue in an atrophic testicle of a patient evincing Turner's syndrome were shown not to contain primary tumor. The excellent therapeutic results confirm the correctness of the combined surgical, radiotherapeutic and chemotherapeutic treatment; it appears that correct timing of the various therapies is of prime importance.