Background: Epidermal growth factor receptor (EGFR) blockade can effectively shrink tumors in metastatic colorectal cancer (CRC) patients without RAS nor BRAF mutations. However, some patients without RAS nor BRAF mutations in their primary tumors have them in metastatic tumors (heterogeneity). Circulating tumor DNA (ctDNA) can reflect these mutations in metastatic tumors. Here, we evaluated the efficacy of EGFR blockade in patients who had no RAS nor BRAF mutations in their primary tumors, but who had them in ctDNA. Methods: We prospectively enrolled 100 patients with confirmed metastatic CRC without RAS nor BRAF mutations in their primary tumors. Patients were treated with first-line systemic chemotherapy that included EGFR blockade. We obtained ctDNA from each patient before they started chemotherapy, and at the time they acquired resistance. We detected RAS, BRAF (V600E), and PIK3CA mutations using digital polymerase chain reaction. Results: In the ctDNA obtained before starting chemotherapy, RAS,BRAF, and PIK3CA mutations were detected in 10, 4, and 2 of the 100 patients, respectively. No patients had both RAS and BRAF mutations in their ctDNA; however, one patient had both BRAF and PIK3CA mutations. Eighty-nine patients had measurable tumor lesions. Of those, 3 experienced a complete response (CR), 72 had a partial response (PR), 12 had stable disease (SD), and 2 had progressive disease (PD). Of 14 patients who had measurable tumors and RAS or BRAF mutations in the ctDNA, 11 (79%) had a PR and 3 (21%) had SD. Of 75 patients who had measurable tumors and no RAS or BRAF mutations in the ctDNA, 3 (4%) experienced a CR, 61 (81%) had PR, 9 (12%) had SD, and 2 (3%) had PD. Patients with RAS or BRAF mutations and patients with neither mutation had response rates of 79% and 85%, respectively. There was no significant difference. Conclusions: As we previously reported, incidence of RAS and BRAF heterogeneity were 10% and 4%, respectively. Heterogeneity of RAS and BRAF mutations had no effect on the response rate of EGFR blockade as first line chemotherapy. Other possible causes of resistance could be MET or HER-2 amplification, PIK3CA mutation, or ALK fusion. The effect of heterogeneity on the duration of response is of great interest; however, one-third of patients are still undergoing treatment with EGFR blockade. Clinical trial information: 000031177.