To explore the clinical significance of the level of HER2/neu gene amplification in a homogenous cohort of 33 patients with HER2-positive metastatic breast cancer (MBC) and available tumor samples treated with a trastuzumab-based regimen, we retrospectively performed dual-color fluorescence in-situ hybridization test and correlated them for each patient with time-to-progression (TTP) and overall survival (OS). We obtained values of HER2/chromosome 17 centromere (CEP17) ratio ranging from 2.5 to 21 (median 7.2). At the Cox model there is indication that patients whose tumors have high-level HER2/CEP17 ratio have shorter TTP and OS than those with lower ratio, when treated with a trastuzumab-based regimen. Correlations do not reach the limits of statistical significance but no formal sample size calculation was performed due to the explorative nature of the study. If confirmed in larger cohorts of patients, HER2/CEP17 ratio could represent a reliable and economical predictor of response to trastuzumab-based therapy in MBC.