Clonal evolution and outgrowth of cellular variants with additional chromosomal abnormalities are major causes of disease progression in chronic lymphocytic leukemia (CLL). Because new DNA lesions occur during S phase, proliferating cells are at the core of this problem. In this study, we used in vivo deuterium ((2)H) labeling of CLL cells to better understand the phenotype of proliferating cells in 13 leukemic clones. In each case, there was heterogeneity in cellular proliferation, with a higher fraction of newly produced CD38+ cells compared with CD38- counterparts. On average, there were 2-fold higher percentages of newly born cells in the CD38+ fraction than in CD38- cells; when analyzed on an individual patient basis, CD38+ (2)H-labeled cells ranged from 6.6% to 73%. Based on distinct kinetic patterns, interclonal heterogeneity was also observed. Specifically, 4 patients exhibited a delayed appearance of newly produced CD38+ cells in the blood, higher leukemic cell CXC chemokine receptor 4 (CXCR4) levels, and increased risk for lymphoid organ infiltration and poor outcome. Our data refine the proliferative compartment in CLL based on CD38 expression and suggest a relationship between in vivo kinetics, expression of a protein involved in CLL cell retention and trafficking to solid tissues, and clinical outcome.