Varicella-zoster virus (VZV) frequently causes severe infections in patients who have undergone bone marrow transplantation. The frequency of, characteristics of, and risk factors for this infection were studied in 164 patients undergoing autologous peripheral blood progenitor cell transplantation (PBPCT). Twenty-six patients (15.8%) developed VZV infection, and the actuarial risk was 10% at 1 year. No patient had visceral dissemination or died because of VZV, although one-third of the patients developed postherpetic neuralgia. By multivariate analysis, a CD4(+) lymphocyte count of <200 cells/microL (P<.0001; odds ratio [OR], 2.0) and a CD8(+) lymphocyte count of <800 cells/microL (P=.0073; OR, 2.0) at day 30 after transplantation were factors associated with VZV infection. Patients with both these adverse factors had an actuarial risk of VZV of 48% at 1 year. Patients with deficiency in both CD4(+) and CD8(+) lymphocytes are at high risk of VZV infection. These patients should be considered as candidates for preventive therapy, but whether for antiviral therapy or vaccination remains to be investigated.