Changes in 2-[18F]-fluoro-2-deoxy-d-glucose (FDG) incorporation by tumors, detected using positron emission tomography, during response to chemotherapy are utilized clinically in patient management. Here, the effect of treatment with growth-inhibitory doses of the anti-human epidermal growth factor receptor-2 antibody trastuzumab (Herceptin) on the incorporation of FDG by breast tumor cells was measured along with hexokinase (HK) and glucose transport to determine the potential of FDG-positron emission tomography in predicting response to these biological anti-cancer therapies and their modulatory effects on the steps involved in FDG incorporation. Methods: The sensitivity to trastuzumab of three breast tumor cell lines, SKBr3, MDA-MB-453 and MDA-MB-468, expressing human epidermal growth factor receptor-2 at high, medium and low levels, respectively, was determined using MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay over a 6-day period, and a clonogenic assay was carried out after 7- and 10-day exposures. FDG incorporation by cells treated with growth-inhibitory doses of trastuzumab was carried out after 4 h and 2, 4 and 6 days of treatment. Glucose transport (rate of uptake of the non-metabolizable analogue [3H]O-methyl-d-glucose), HK activity and lactate production were measured on cells treated with inhibitory doses of trastuzumab for 6 days. Results: The IC50 doses for SKBr3 and MDA-MB-453 and the IC20 dose for MDA-MB-468 after 6 days of treatment with trastuzumab were 0.25, 1 and 170 µg/ml, respectively. FDG incorporation by SKBr3 and MDA-MB-453 cells was found to be decreased using IC50 doses of trastuzumab for 6 days. At the IC50 doses, FDG incorporation was also decreased at 4 days and, in the case of MDA-MB-453, even after 4 h of treatment. Decreased FDG incorporation corresponded with decreased HK activity in these cells. Lactate production, previously suggested to be a potential measure of response, was found to be significantly decreased by SKBr3 and MDA-MB-453 cells responding to trastuzumab. Conclusion: FDG incorporation at the tumor cell level is modulated by treatment with growth-inhibitory doses of trastuzumab due to modulation of HK activity. Changes in lactate production may also be a useful determinant of response to trastuzumab. © 2010 Elsevier Inc. All rights reserved.