Diencephalic syndrome is characterized by excessive weight loss and subcutaneous fatty tissue loss, hyperactivity, euphoria, and hypervigilance observed in early childhood. It usually accompanies space-occupying neoplastic lesions of the hypothalamic–optochiasmatic region. There is an inverse relationship between mass size and weight gain. In this case report, a patient aged 1 year and 2 months with hypothalamic–optochiasmatic glioma was presented. We aimed to draw attention to diencephalic syndrome, which is a rare cause of weight gain and vomiting.