Small numbers of monoclonal B cells with the phenotype of chronic lymphocytic leukemia (CLL) were detected by means of high-sensitivity flow cytometry in about 5% of adults with a normal blood count and about 14% of subjects with lymphocytosis. In the latter group, frank CLL ultimately developed in 15%. The principal features of these monoclonal B cells and the B cells of CLL were similar. Small numbers of monoclonal B cells with the phenotype of chronic lymphocytic leukemia (CLL) were detected by means of high-sensitivity flow cytometry in about 5% of adults with a normal blood count and about 14% of subjects with lymphocytosis. In the latter group, frank CLL ultimately developed in 15%. The incidence of chronic lymphocytic leukemia (CLL) is approximately 6 per 100,000 persons per year in North America. 1 The National Cancer Institute–sponsored Working Group guidelines for the diagnosis of CLL require a lymphocyte count of 5000 or more per cubic millimeter and a characteristic cell-surface phenotype of B cells: the presence of CD19, CD5, and CD23, weak expression of CD20 and CD79b, and either kappa or lambda immunoglobulin light chains. 2 High-sensitivity flow cytometry allows for the detection of B cells with a CLL phenotype in numbers as low as 1 per 10,000 normal leukocytes. 3 With this method, CLL-phenotype cells have . . .