Nineteen of 160 children developed varicella between eight days and 6.4 years following renal transplantation. Eight had severe varicella characterized by prolonged fever and new vesicle formation with rash involving mucous membranes. The severe group had an increased incidence of thrombocytopenia and markedly elevated liver enzyme values. Two patients of this group had bladder paralysis and another died. In three children the post-transplant varicella represented a second attack of this disease. Children maintained on azathioprine therapy for three days or more after onset tended to have severe varicella. No graft loss occurred consequent to stopping azathioprine. Children with transplants at risk should have zoster immune plasma or globulin upon exposure, and azathioprine therapy should be stopped at onset of varicella. Corticosteroid therapy should be continued in order to avoid stress-induced Addisonian crisis.