Subtle immunodeficiency to infectious agents including measles virus and ten Epstein-Barr virus (EBV) has been described in the X-linked recessive lymphoproliferative syndrome. This syndrome has affected six male cousins and possibly another boy. Three brothers died of an infectious mononucleosis syndrome, in a maternal cousin agammaglobulinemia developed three years after infectious mononucleosis, and two half-brothers of the Duncan kindred died of lymphoma of the brain and intestinal tract, respectively. In three of the boys, unusual measles viral infections had developed. Paramyxovirus-like particles suggestive of measles virus were seen at necropsy in the atrophic lymphoid tissue of two boys. Also, numerous plasma cells were seen in the brains, visceral organs and the thymus glands, and thymic-dependent lymphocytes were sparse in lymph nodes and spleen. The abnormal lymphopoiesis in the syndrome probably results from a subtle immunodeficiency, and concurrent measles and EB virus infections.