• Five patients with radial aplasia studied during a period of ten years at the University College Hospital, Ibadan, Nigeria, had the amegakaryocytic thrombocytopenia syndrome. Together, they are termed the TAR syndrome. Two of the five patients were siblings from a polygamous family; three patients had relative hypogammaglobulinemia, and one of these patients had late onset of congenital varicella. The findings in these five cases led to further understanding of the genetic mechanisms of the TAR syndrome. Furthermore, the high incidence of infection associated with the syndrome could be partially explained by hypogammaglobulinemia.