Abstract Background Identifying predictive biomarkers is important to optimally treat patients. This analysis evaluated the association of K- ras , BRAF , and PIK3CA gene mutations with tumor resistance to panitumumab alone. Patients and Methods From 3 phase II panitumumab metastatic colorectal cancer (mCRC) studies, 62 of 533 patient samples were available. Mutations were identified from genomic DNA by sequencing. Results Of the 62 samples, 24 (38.7%) harbored a K- ras mutation, and 38 (61.3%) were wild type. In the wild-type K- ras group, 11% of patients had a partial response (PR), 53% had stable disease (SD), and 37% had progressive disease (PD). In the mutant K- ras group, 21% of patients had SD, and 79% of patients had PD; there were no responses. The absence of a K- ras mutation was associated with response to panitumumab (PR vs. SD vs. PD; P = .0028). The hazard ratio for wild-type versus mutant K- ras was 0.4 (95% CI, 0.2-0.7) for progression-free survival and 0.5 (95% CI, 0.3-0.9) for overall survival. Four patients had a V600E BRAF mutation, and 2 patients had a PIK3CA mutation. Conclusion These data suggest that patients with mCRC with activating K- ras mutations are less likely to respond to panitumumab alone. The small sample size limits us from defining a predictive role of PIK3CA and BRAF mutations for panitumumab treatment.