Among 111 immunocompetent patients referred to a general hospital setting with the clinical diagnosis of herpes zoster, viral cultures were obtained from 47 patients. Six of these patients (13 percent) had herpes simplex virus isolated, with four of the six infections involving the facial distribution, and the other two involving the T4 (breast) distribution. Excluding those in whom herpes simplex virus was isolated, the mean age (+/- SD) of the remaining 105 patients was 50 +/- 19 years. Thirty-two percent of the patients were at least 65 years old; however, 39 percent were younger than 40 years of age. Thus, herpes zoster frequently occurs in young, immunocompetent adults. Also, since zosteriform rashes may be caused by herpes simplex virus, viral cultures of lesions are useful to differentiate infections caused by herpes simplex virus from those due to varicella-zoster virus. The need to distinguish between these two viruses may be important with the advent of antiviral drugs and for use of the proper epidemiologic isolation procedures.