Hairy cell astrocytoma is a grade I glioma, or what we often call a benign glioma. These tumors are common in children, and the two common sites are the saddle area and the brainstem. Hairy cell astrocytomas in the saddle area are mostly optic pathway gliomas. Because they affect the feeding center, children are very thin and can be recognized immediately in the outpatient clinic or in the ward. The weight will rise quickly and dramatically after surgery. Another site is the brainstem, mostly with cystic degeneration, as in the case discussed today. Hairy cell astrocytomas are pathologically very good looking tumors with wheal-like bipolar cells. The pathologist also focused on its pathologic features today. As for the treatment, surgery is the first option, with as complete resection as possible and no postoperative treatment. However, for various reasons, residual tumor was finally chosen as a last resort in order to preserve neurological function. Optic pathway glioma is sometimes chosen for tumor residual in order to preserve weak vision and brainstem hair cells in order to protect brainstem function. Postoperatively, radiotherapy is performed when conditions allow, and such condition is that the residual tumor has a better boundary with the hypothalamus or brainstem, otherwise radiotherapists are also discouraged.