Herpes simplex encephalitis (HSE) is the most common cause of sporadic focal encephalitis worldwide. Acyclovir is the treatment of choice of HSE since the 1980s. After the widespread use of acyclovir, HSE related mortality rate had reduced but resistant strains emerged. Acyclovir resistant HSV incidence was reported as about 0.5 % and 3.5 %-10 % in immunocompetent and immunocompromised patients, respectively. Herein, a 12-year-old immunocompetent patient with HSV-1 encephalitis who was successfully treated with combined acyclovir and foscarnet therapy is described. In the case of deteriorating clinical condition under acyclovir treatment even if the absence of demonstration of increased CSF HSV viral load, the possibility of acyclovir resistant HSE and the addition of foscarnet to the acyclovir treatment might be considered.