Herpes zoster (HZ), a varicella-zoster virus reactivation, frequently complicates hematopoietic stem cell transplantation (HSCT). Its incidence, complications, and associated risk factors in 310 children undergoing HSCT were reviewed. In all, 61 of 201 (32%) patients who had undergone allogeneic and 10 of 109 (9%) patients who had undergone autologous HSCT developed HZ. Of 90 VZV seropositive allogeneic patients, 50 (53%) developed HZ. Seven (17%) of 41 VZV seropositive autologous patients developed HZ. Although a substantial number of patients develop HZ in the early post-HSCT period, risk for HZ persists and HZ can occur up to 5 years post-HSCT. Risk factors for HZ included age 410 years (P < 0.0001), allogeneic HSCT (P < 0.001), and total body irradiation (TBI) (P < 0.059) in allogeneic recipients. Of 37, 22 (59%) patients experienced an elevated alanine aminotransferase (ALT), unassociated with GVHD, in the month preceding HZ. Of the 48/64 patients (75%) hospitalized for treatment ( median stay, 6 days; range, 2-39), length of stay was unaffected by donor type but increased by cutaneous dissemination and visceral involvement (P = 0.023 and 0.034, respectively) in allogeneic patients. Consideration of HZ infection particularly in patients 410 years of age with elevated ALT after TBI-conditioned allogeneic HSCT may permit earlier diagnosis and therapeutic intervention.