The purpose of this study was to characterize varicella in childhood cancer patients. Seventeen of the 77 patients reviewed were in remission and off all therapy for 3 to 22 months. No one in this group died from varicella or had evidence of visceral dissemination. Among the remaining 60 patients, all of whom were receiving anticancer therapy when they developed vancella, 19 (32%) had visceral dissemination and 4 died, for a mortality rate of 7%. Each of the deaths was associated with primary varicella pneumonitis, with or without acute encephalitis. Visceral dissemination was not related to type or status of malignancy or to duration of specific anticancer therapy. Varicella was more likely to disseminate in children with absolute lymphopenia, less than 500 cells per cubic millimeter, than in patients with higher lymphocyte counts. Cessation of anticancer therapy prior to the onset of lesions appeared to lessen the risk of dissemination. These results show that varicella is more severe in cancer patients on therapy than in the general population or in patients who have completed therapy, but is not highly fatal.