HSV causes serious complications in immunocompromised patients, especially SCT recipients. Although ACV is an effective antiviral prophylaxis, the emergence of ACV resistance is a growing problem. The authors describe two cases of fatal ACV-resistant HSV in two pediatric patients following unrelated donor SCT. Despite the in vitro sensitivity of the HSV isolates to foscarnet, both patients failed to respond to foscarnet therapy. Other antiviral therapies should be considered in those patients who fail to show rapid clinical improvement.