Appearance of residual masses following chemotherapy of retroperitoneal metastasis from testicle's non-seminomatous germinal cell tumour does not always imply we are facing an active tumoral disease. When exeresis of these masses discovers the presence of a mature teratoma during the histological study, what we have is the so called Growing Teratoma Syndrome. Such teratoma is unresponsive to chemotherapy but the patient can be cured by means of a full exeresis of the residual mass, thus the relevance of having into account the likelihood of this pathology. The paper presents the case of a patient with retroperitoneal non-seminomatous testicle tumour and metastasis which, following chemotherapy, presented a residual mass with normal serological markers. After the exeresis it was found out that it was made up by mature teratoma. Four months after rescue surgery, the patient remains asymptomatic, has normal markers and no signs of neoplastic recurrence.