One hallmark of tumor cells is a high rate of glycolysis, which underlies the utility of the glucose analog, 2-fluoro-2-deoxyglucose for PET imaging of solid tumors. The glucose analog, 2-deoxyglucose (2DG), inhibits glycolysis, and is predicted to decrease the viability of tumor cells, which are primarily dependent on the glycolytic pathway for ATP generation. We have recently shown that 2DG enhances the efficacy of either adriamycin or paclitaxel in delaying tumor growth in mouse xenograft cancer models (Maschek et al, Cancer Res 64 (2004): 31). We are currently conducting a phase 1 clinical trial to examine the safety of a combination administration of 2DG and Taxotere in patients with solid tumors. The goal of the current study is to measure the extent to which 2DG inhibits energy production and, consequently, reduces viability of leukemic cells. Heparinized blood was obtained from four subjects with a confirmed diagnosis of CLL (peripheral lymphocyte blood count ranged from 5,000–24,000/mm3), with an additional sample drawn from each subject two weeks later for replicate determinations. The mononuclear cell fraction (predominantly B cells) was isolated using Histopaque. Subsequent treatment of isolated CLL cells with 2DG (164 μg/ml) at 37°C (in the presence of 0.5 mM glucose) resulted in a 20–30% decrease in cellular ATP levels after 1 hr (compared with untreated cells) and a 60–70% decrease in ATP levels after 24 hrs. In the presence of 5 mM glucose, incubation of CLL cells with 2DG elicited a 5–10% decrease in ATP at 1 hr and 15–20% reduction in ATP levels at 24 hrs. The effect of 2DG on cell viability was assessed using Alamar Blue. Treatment of CLL cells with 2DG (164 μg/ml, in the presence of 0.5 mM glucose) resulted in a 50% loss in cell viability at 24 hrs (compared with untreated cells at the same time point), and a 70% loss at 48 hrs. In the presence of 5 mM glucose, the effect of 2DG was more modest. At 24 hrs, there was a 20% loss of cell viability and a 30–50% loss at 48 hrs. In conclusion, these results demonstrate that in vitro treatment of CLL B cells with 2DG decreases cellular ATP levels, ultimately resulting in a loss of cell viability. Updated results with samples from additional CLL subjects will be presented, together with an assessment of the therapeutic potential of 2DG for the treatment of CLL.