Herpes simplex encephalitis (HSE) is a life-threatening condition with high mortality. The pathogenesis underlying the reactivation of latent herpes simplex virus (HSV) remains undefined. We present the case of a 55-year-old female who developed HSE type 1 during brain irradiation and antioedematous dexamethasone treatment for leptomeningeal metastasized breast tumor with epileptic seizures. During the radiotherapy (RT), after a total of 32 Gray administrated in 16 fractions, our patient developed cognitive impairment and partial epileptic status without fever. Two days later the patient's clinical conditions had deteriorated and high fever manifested. A diagnosis of HSE type 1 was made by a positive cerebrospinal fluid polymerase chain reaction. Antiviral therapy with high doses of acyclovir was practiced for four weeks but the comatose state persisted. The patient died 59 days after the last RT fraction. The temporal relationship of RT to the occurrence of HSE suggests that cranial irradiation may play a role in the reactivation of latent HSV. Although antiviral therapy resistance is infrequent in immunocompetent patients, it is one of the main problems in immunocompromized patients.