Overexpression of the HER2/neu oncogene and receptor protein has been reported in 20%-30% of patients with breast cancer and is associated with a poor prognosis. HER2/neu expression in breast cancer patients assessed by fluorescence in situ hybridization or immunohistochemistry is a predictor for response to trastuzumab, a humanized monoclonal antibody against the HER2/neu cell-surface protein. Data regarding HER2/neu expression in lung cancer are more limited, and there is little information regarding HER2/neu expression and response to trastuzumab alone or in combination with chemotherapeutic agents. Gemcitabine is an active agent against non-small-cell lung cancer (NSCLC) and has demonstrated activity in breast cancer as well. In vitro modified tetrazolium salt growth assays were performed to determine whether the combination of trastuzumab/gemcitabine produced synergistic or additive effects on breast and lung cancer cell lines. The effects of trastuzumab alone, gemcitabine alone, and the trastuzumab/gemcitabine combination was evaluated on 4 NSCLC cell lines, 1 small-cell lung cancer (SCLC) cell line, and 2 breast cancer cell lines. HER2/neu surface protein expression was assessed by fluorescence flow cytometry and immunohistochemistry. Fluorescence in situ hybridization analysis was used to study gene expression. Trastuzumab treatment alone resulted in growth inhibition in all cell lines expressing HER2/neu and the inhibitive effect correlated with the level of cell surface HER2/neu protein expression. Treatment with gemcitabine alone resulted in growth inhibition in both breast and NSCLC cell lines. A synergistic growth inhibition effect was seen with the trastuzumab/ gemcitabine combination as indicated by combination index values < 1. The degree of synergy observed did not directly correlate with the level of surface protein expression, as synergy was seen even in cancer cell lines expressing low levels of HER2/neu. No treatment effect was seen in the SCLC cell line, which did not express HER2/neu. These preclinical studies indicate a need to study the clinical synergistic effects of the gemcitabine/trastuzumab combination in breast cancer and NSCLC patients whose tumors overexpress HER2/ neu.