INTRODUCTION. Monoclonal B cell lymphocytosis (MBL) refers to a monoclonal population of B lymphocytes <5000 cells/microL (<5 x 10 9 /L) in peripheral blood for ≥3 months, without other features of a B cell lymphoproliferative disorder (eg, lymphadenopathy, organomegaly, cytopenias, or extramedullary involvement) [].. This topic will discuss the epidemiology, evaluation, diagnosis, prognosis ...