Stimulation of lymphocytes by varicella-zoster virus (VZV) antigen was measured in vitro for 37 healthy adults and children and for 35 children who developed herpes zoster or varicella while receiving anticancer therapy. For 35 of the control group the history of varicella infection correlated with the in vitro response of lymphocytes to VZV antigen. A specific lymphocyte response was observed following 32 of the 37 episodes of VZV infection in the 35 children with malignancy. Re-evaluation 7--12 months following the acute infection revealed that the in vitro response to VZV antigen was lost in six of 19 patients who remained in remission and in all patients who relapsed and received reinduction therapy. Although there was some suggestion that development of a cell mediated response early in the infection decreased the incidence of skin dissemination of the virus in herpes zoster, no consistent correlation could be made between lymphocyte responsiveness in vitro and dissemination of disease or the duration of the appearance of new lesions.