Immature teratoma (IMT) is normally treated by resection and adjuvant therapy. The present unusual case of recurrent germinoma occurred 21 years after total resection of pineal IMT. A 3-year-old boy presented with headache, disturbance of consciousness, and Parinaud’s syndrome. Magnetic resonance (MR) imaging revealed a pineal mass lesion, and total resection of the tumor was achieved. The histological diagnosis was mature teratoma. He did not receive further treatment, and did well without recurrence for 20 years. However, he suffered headache 21 years after resection, and MR imaging revealed a homogeneously enhanced pineal mass with low minimum apparent diffusion coefficient value and proton MR spectroscopy showed a huge lipid peak. The levels of tumor markers were not elevated. Cerebrospinal fluid (CSF) cytology found atypical cells with large nuclei and irregularly shaped nucleoli. To elucidate the relationship between the primary and recurrent tumors, we reviewed the histological specimens and CSF cytology at the initial treatment and found a subset of incompletely differentiated components resembling fetal tissues in the histological specimen and atypical large cells in the CSF. Based on these radiological and histological findings, we presume that the recurrent disease was disseminated germinoma after the resection of disseminated IMT. He received chemotherapy and craniospinal radiation therapy, and the enhanced lesion and atypical cells in the CSF disappeared. This case demonstrates that disseminated IMT can be controlled for the long term without adjuvant therapy, but may recur as germinoma. Tumor dormancy may account for this unusual course.