Herein, safe and reliable neuroendoscopic biopsy via an extremely narrow foramen of Monro (ENFM) for a non-hydrocephalic patient with hypothalamic and pineal region tumors was successfully applied. A 17-year-old boy presented with hypothalamic manifestations attributed to hypothalamic and pineal region tumors. Small ventricles were seen. Intraoperatively, to advance different diameter steerable fiberscopes via ENFM, the third ventricle was flushed to induce a moment increase in the intraventricular pressure with subsequent dilatation of FM. Postoperative course was uneventful. Histopathological studies revealed a yolk sac tumor. Adjuvant therapy was applied. Follow-up neuroimaging disclosed marvellous improvement of the condition. His symptoms gradually improved.