A 4-year-old girl who had received a fetal thymus gland by intraperitoneal transplantation 41 months previously sustained acute, fatal bronchiolitis due to culture-proven cytomegalovirus despite the fact that a specific antibody response to this organism was detected. While the thymic transplantation had increased the number of circulating T lymphocytes and had permitted immune sensitization to delayed-hypersensitivity skin test antigens, there was still an incomplete state of T lymphocyte function. In particular, isolated lymphocytes failed to respond to stimulation with phytohemagglutinin at several concentrations and, more important, the pathologic examination demonstrated a severe anatomic deficiency of lymphoid tissue associated with T lymphocyte function. The unusual infection that caused the death of this child emphasized the necessity of acquiring sufficient T lymphocyte function in immunologic reconstitution attempts.