Context.-The therascreen KRAS RGQ polymerase chain reaction kit is being developed as a companion diagnostic to aid clinicians, through detection of KRAS mutations, in the identification of patients with metastatic colorectal cancer (mCRC) who are more likely to benefit from cetuximab. Objective.-To assess whether KRAS mutation status, determined by using the therascreen KRAS kit, is a predictive marker of cetuximab efficacy. Design.-Tissue samples were obtained from patients with mCRC treated on the National Cancer Institute of Canada Clinical Trials Group (NCIC CTG) CO.17 phase 3 study of cetuximab plus best supportive care (BSC) versus BSC alone. Tumor DNA samples were assessed for the presence of KRAS mutations by using the therascreen KRAS kit. Efficacy and safety were assessed to determine whether mutation status was predictive of outcomes. Results.-Evaluable samples were available from 453 patients (79.2%) enrolled in the NCIC CTG CO.17 trial. The KRAS wild-type subset represented 54.1% (245 of 453) of the evaluated population. Median overall survival of patients with KRAS wild-type tumors was 8.6 months among those who received cetuximab plus BSC and 5.0 months among patients who received BSC alone (hazard ratio [HR], 0.63; P.002). Among patients with KRAS mutant mCRC, no meaningful difference in overall survival was observed between arms (HR, 0.91; P.55). These results are consistent with a previous report that analyzed patient tumor samples by using bidirectional sequencing. Conclusions.-These data support the utility of the therascreen KRAS kit as a means of selecting patients who may benefit from cetuximab therapy.