Presence of retroperitoneal or mediastinal mature teratomas secondary to metastatic testicle germinal tumours which have not responded to multiple chemotherapy constitute the so called Growing Teratoma. They can either represent the metastasis of a mature teratoma site in the original testicle tumour or the evolution from a non-differentiated lesion during therapy. The first two cases of Growing Teratoma Syndrome published in the national literature are presented here. Both cases had a retroperitoneal growth of the mature teratoma following successful achievement of normalization of serum tumoral markers through chemotherapy with CDDP, Bleomycin, and VP-16 in one case (Hospital Clínico) and with BOMP/EPI (Belomycin, Metrothexate, Vincristine, Cisplatin/Vo-16, Cisplatin, Iphosphamide) in the second one (Hospital del Mar) of a non-seminomatose testicle tumour. After a follow-up of 14 and 9 months respectively the good prognosis of these mature teratomas was verified in both cases, so as in the rest of the literature. We coincide with regard to performing a retroperitoneal lymphadenectomy with exeresis of tumoral masses as the most adequate attitude, both to prevent growth of the mature teratoma and to allow us to discard the presence of residual immature lesions.