Introduction Advances in systemic chemotherapy, molecular targeted therapy and immunotherapy have extended and improved the quality of life of patients with cancer. However, the central nervous system is very susceptible to complications of systemic cancer and its treatment. Posterior reversible encephalopathy syndrome (PRES) is a rare clinical and neuroradiologic entity which has garnered increasing recognition in the past two decades. Cancer patients are generally treated with cytotoxic agents, immunotherapy, molecular targeted therapies or glucosteroids which are more frequently associated with PRES. Case presentation A 59-year old female, known with a relapse of her lung adenocarcinoma, had been treated with 4 cycles of cisplatin (75 mg/m²) and pemetrexed (500 mg/m²). Six weeks after this combination chemotherapy and within 28 h after the administration of pemetrexed maintenance therapy, she developed a generalised epileptic insult. Magnetic resonance imaging (MRI) of the brain showed bilateral areas of increased signal intensity in the subcortical parietal and frontal white matter. She was treated with a broad spectrum antiseizure drug, levetiracetam 750 mg twice daily and strict control of blood pressure. Discussion Diagnosis of PRES should be considered in all patients with neurologic symptoms who are at risk to develop PRES. It is crucial to establish the diagnosis as soon as possible since there is no specific treatment of PRES other than correction of the underlying risk factors and preventing seizure recurrence. Administration of pemetrexed is a possible risk factor for the development of PRES.