BACKGROUND: We report two cases of diffuse cerebellar swelling with upward transtentorial herniation following medulloblastoma resection. We present our insight for managing medulloblastoma with aggressive clinical behavior. Case 1 is a 32-month-old boy. Magnetic resonance imaging (MRI) revealed fourth ventricular mass with diffuse leptomeningeal metastasis. He underwent gross total resectioning of the tumor and histopathology revealed a large cell medulloblastoma. A ventriculoperitoneal shunt was placed on the 25th postoperative day. Ten days after he developed lethargy, and MRI showed diffuse cerebellar swelling with upward herniation. He underwent emergent posterior fossa decompression; however, he remained unresponsive since then. Case 2 is a 31-month-old boy. MRI revealed a fourth ventricular mass with diffuse leptomeningeal metastasis. He underwent gross total resection and histopathology was a large cell medulloblastoma. Due to developing ventriculomegaly, extraventricular drainage was reinserted on the 11th postoperative day. Four days after, he developed lethargy and decerebrate posturing. MRI showed diffuse cerebellar swelling with upward herniation. He underwent aggressive posterior fossa decompression. Chemotherapy was started postoperatively. Gradually, his movement of extremities improved. On the 79th postoperative day, he started to follow commands and talk some words. He was discharged to a rehabilitation institute and has continued to improve since then. CONCLUSION: Diffuse cerebellar swelling with upward herniation could occur in patients with aggressive medulloblastoma postoperatively. Our patients had diffuse leptomeningeal dissemination of the tumor at the initial presentation. Close monitoring and prompt diagnosis with earlier surgical posterior fossa decompression and administration of chemotherapy may prevent irreversible neurologic deterioration.