Persistent polyclonal B-cell lymphocytosis in identical twins.

This is the first report of the unusual syndrome of persistent polyclonal B-cell lymphocytosis occurring in monozygotic twins. The syndrome is characterized by a lymphocytosis, with circulating atypical, binucleated lymphocytes, mild splenomegaly and raised serum IgM. It occurs predominantly in females, with serological evidence of previous EBV infection, and is associated with cigarette smoking and HLA-DR7 phenotype. The association with .DR7 suggests a genetic predisposition. Its occurrence in identical twins, documented here, provides stronger support for a hereditary/genetic basis for the syndrome.