Prophylactic cranial irradiation (PCI) should now be considered as a part of the standard treatment of patients with small cell lung cancer (SCLC) in complete remission. The PCI has been offered in SCLC to reduce the incidence of brain metastasis and increase survival. The complications of PCI were reported brain necrosis, seizure or dementia. The complications were more frequent when chemotherapy was given at the time of cranial irradiation, or large radiation fraction size was employed. It is established that the pathophysiological reaction to irradiation in the normal brain tissue is necrosis, demyelinization, and diffuse changes due to wall thickening of the vascular structures. However, central pontine myelinolysis (CPM) of low dose irradiation like PCI is very rare. We report a patient with the classical syndrome of CPM following PCI for SCLC. The diagnosis was supported by typical features on magnetic resonance imaging.