A CD5(+)23(+) monoclonal B-cell population is detectable in approximately 3% of the general adult population. The phenotype of the monoclonal CD5(+)23(+) B cells is identical to chronic lymphocytic leukaemia (CLL) with respect to a large number of proteins in addition to the standard diagnostic markers used to identify CLL. Studies in CLL families and direct assessment of genetic features indicate a close biological association between indolent CLL and the CLL-phenotype cells detected in individuals with a normal blood count. Patients with a CLL-phenotype monoclonal B-cell lymphocytosis (MBL) often have increasing CLL cell counts with time and some progress to a stage requiring treatment. Analysis of intraclonal variation in the immunoglobulin heavy chain gene suggests a process of clonal diversification rather than clonal selection in the early stages of disease progression. CLL-phenotype MBL is detectable in approximately 10% of cases referred for investigation of a lymphocytosis and future studies should be directed towards the detection of factors which identify MBL patients at risk of disease progression.