This report describes a mixed germ cell tumor that underwent dramatic size changes. A 12-year-old boy presented to our hospital with a headache that had persisted for two months. Initial magnetic resonance imaging (MRI) revealed a pineal tumor and hydrocephalus. The patient required external ventricular drainage and underwent two endoscopic biopsies. His evaluation involved a total of nine computed tomography (CT) scans prior to the second biopsy;the tumor size had decreased before the second endoscopic biopsy. The tumor consisted of both a germinoma and a teratoma component. The patient was treated with three courses of carboplatin-etoposide (CBDCA-VP) chemotherapy and whole-ventricle radiotherapy (32.1 Gy). However, during the adjuvant therapy, the tumor size increased, necessitating total tumor resection. We speculate that the tumor's initial size reduction was caused by leakage of the cyst component and exposure to the brain CT irradiation. The tumor's subsequent increase in size was due to the recollection of the cystic components and intracranial growing teratoma syndrome (iGTS). Therefore, frequent brain CTs and angiography should be avoided before definitive pathological diagnosis is achieved. Further, the tumor size should be considered, with surgical resection being performed at the optimal time.