Background: Although KRAS and EGFR extracellular domain acquired mutations were detected in two small cohorts and correlated with acquired resistance to anti-EGFR monoclonal antibodies (MAb), the frequency, co-occurrence, and distribution of these acquired mutations is unknown. In this study we evaluated the presence of acquired KRAS and EGFR mutations in cfDNA from CRC patients (pts) treated with anti-EGFR monoclonal antibody. Methods: Plasma was collected from EGFR-MAb refractory mCRC pts as part of the ATTACC (Assessment of Targeted Therapies Against Colorectal Cancer) program. Eligible pts had documentation of pre-treatment KRAS wild type tumor. The cfDNA was extracted from the plasma and analyzed by BEAMing technology for acquired KRAS and EGFR mutation. Results: The plasma from 55 patients was analyzed for EGFR and KRAS mutation. The S492R EGFR mutation was detected in 4 pts (7%) treated with cetuximab. Acquired KRAS mutations were detected in 26 of the 55 KRAS wt samples analyzed (47%). Although codon 61 and 146 mutations are rare in untreated CRCs (2% and 1% of the MDACC population, respectively), these atypical KRAS mutations predominated in acquired resistance (Q61H=33% and A146T=10%). Mutations in more than one KRAS codon are exceedingly rare in the primary tumor. In our study we detected more than one KRAS or EGFR mutation in 30% of the population (p<0.001), suggesting the development of multiple independent clones in individual patients. Compared to 8 patients with known KRAS mutations, the average number of mutant reads in the 26 patients with acquired mutation was substantially lower (p<0.01) despite similar tumor burden. Of note, acquired concomitant KRAS mutations were also found in a BRAF V600 mutant patient previously treated with anti-EGFR MAb and a BRAF inhibitor. Conclusions: KRAS and EGFR acquired mutation are present at low concentrations in cfDNA from mCRC pts refractory to anti-EGFR MAb and they are not mutually exclusive, suggesting heterogeneity of the resistant clones. Anti-EGFR MAb refractory patients showed a higher incidence of atypical KRAS mutation and higher incidence of multiple codon KRAS mutations compared with overall CRC patient.