The addition of intrathecal methotrexate to treatment protocols has increased survival rates in children with acute lymphoblastic leukemia but is also associated with varying degrees of neurotoxicity. We describe a 15-year-old female patient diagnosed with acute lymphoblastic leukemia presenting with status epilepticus after receiving intrathecal methotrexate. Magnetic resonance imaging showed reversible cortical and subcortical changes consisting of high-intensity lesions on T2-weighted and fluid-attenuated inversion recovery sequences with postgadolinium enhancement, low signal intensity on diffusion-weighted imaging and increased apparent diffusion coefficient. These findings were consistent with the posterior reversible encephalopathy syndrome. We report our conventional magnetic resonance and diffusion-weighted imaging findings and briefly discuss the pathophysiology of the syndrome.