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Previous work has suggested a relationship between disease activity and in vivobirth rates of chronic lynphocytic leukemia (CLL) cells, as measured by incorporation of deuterium into DNA during heavy water (2H2O) consumption. By extension, CLL-cell kinetics may be a useful prognostic marker in early stage disease, with an increased birth rate predicting shorter time to disease progression and poor prognosis. We are conducting a multi-center study to evaluate the relevance of proliferation rates to both known prognostic markers and to disease progression during 2.5 years of clinical follow up in previously untreated patients with early stage disease. Patients are given a daily dose of 2H2O for 6 weeks and CLL-cell birth rates are measured. We examined the leukemia cells of 36 patients for known prognostic markers (ZAP 70 expression, CD38 expression, IgVH mutation status) and CLL birth rates. Subjects were Rai Stage 0 (n=15), 1 (n=17), or 2 (n=4); mean age 57 +/− 9 (range 41–78); mean time from diagnosis to enrollment 2.1 +/− 2.2 y (range 0.1–10 y). ZAP-70 expression (20% cutoff) was positive in 44%, CD38 expression (30% cutoff) was positive in 35%, and use of unmutated IgVH genes was found in 45% of the subjects. CLL-cell birth rates ranged from 0.09 to 0.97% per day. Utilizing the previously published cut-off value of 0.35% newly divided cells per day, 33% of subjects had a high proliferation rate. Data on all biomarkers were available for 33 subjects. Nine subjects had 4/4 favorable prognostic indicators and 5 subjects had 4/4 unfavorable prognostic indicators. Agreement between prognostic markers, as assessed by kappa coefficient, was strongest for the ZAP-70 v. IgVH mutational status comparison. Comparisons were also significant for ZAP-70 v. CD38 and IgVH mutational status v. CD38. Birth rate was not significantly correlated with any of the other prognostic markers, suggesting it may be a useful independent predictor of disease progression. Isolation of CLL-cells from the bone marrow at the end of the 6 wk labeling period in 5 subjects revealed matching proliferation rates in the marrow and blood in 3 subjects. In 2 subjects, marrow enrichment was lower, suggesting a more slowly proliferating pool in the marrow, the pathophysiologic significance of which is unclear. In 5 subjects, kinetic analysis revealed a very delayed entry of newly divided cells into the blood compartment, presumably from tissue sites of proliferation. This parameter may reflect pool size of CLL-cells in tissues and correlation with lymph node burden is being evaluated. In summary, analysis of CLL-cell birth rates in vivoin these early stage patients provides unique information about the biology of CLL. Correlations with disease progression during the follow-up period will help determine the utility of this test as a prognostic biomarker.