A six-month-old male infant, whose elder brother had died of progressive vaccinia due to combined immunodeficiency, contracted varicella-zoster infection, and died of disseminated varicella ten days after onset of the disease. As with his elder brother, the blood levels of immunoglobulins were normal, and specific varicella antibodies appeared in his serum, although in low concentrations. His T-lymphocytes, although their number was subnormal, could be stimulated with phytohaemagglutinin, and production of migration inhibitory factor could also be demonstrated. The necropsy confirmed thymic dysplasia. In addition to histological changes, severe combined immunodeficiency and foci of malignant lymphoma were present. On this basis the disease was classified, according to the WHO recommendations, as severe combined immunodeficiency with B lymphocytes, complicated by malignant lymphoma.