Eight childhood cancer patients with herpes zoster were serially tested for the presence of varicella-zoster virus in blood. Cell cultures of leukocyte-rich plasma from four patients were positive for the virus. In this study viremia was clearly related to dissemination of dermal lesions-the spread of zoster lesions outside an infected dermatome. The child with the longest viremic phase, five days, had the longest and most severe course of skin dissemination, as well as biochemical evidence of hepatitis. One patient with viremia had advanced embryonal carcinoma and died of disseminated tumor before her clinical course could be evaluated. These observations, the first to document a viremic phase for herpes zoster in immunosuppressed children, furnish an added criterion for evaluation of antiviral drugs and live-virus vaccines in the treatment and prevention of varicella-zoster infections.