In a previous cohort study, we proposed that responsiveness of metastatic colorectal cancer (mCRC) to anti-epidermal growth factor receptor (EGFR) monoclonal antibodies has a genetic basis, being associated with increased EGFR gene copy number (GCN) as measured by fluorescence in situ hybridization (FISH) in individual tumors. The present study was aimed at assessing the predictive role of EGFR GCN, in terms of clinical outcome, in patients treated with panitumumab. Patients with mCRC refractory to standard therapies were a subset of patients from a phase III trial of panitumumab plus best supportive care (BSC; n = 58) versus BSC alone (n = 34) who were selected on the basis of availability of tumor samples adequate for FISH. In patients treated with panitumumab, a mean EGFR GCN of less than 2.5/nucleus or less than 40% of tumor cells displaying chromosome 7 polysomy within the tumor predicted for shorter progression-free survival (PFS; P = .039 and P = .029, respectively) and overall survival (P = .015 and P = .014, respectively). None of the treated patients with mean EGFR GCN of less than 2.47/nucleus or less than 43% of tumor cells displaying chromosome 7 polysomy obtained objective response compared with six of 20 and six of 19 patients with values greater than these cutoff limits, respectively (P = .0009 and P = .0007, respectively). Evaluation of BSC-treated patients showed no correlation between EGFR GCN or chromosome 7 polysomy status and PFS. In a larger and more homogeneous series than in previous studies, present exploratory data suggest that mCRC patients with tumors distinguishable by FISH analysis of EGFR as homogenously disomic or with low chromosome 7 polysomy have a reduced likelihood of response to panitumumab.