Background: Early tumor shrinkage (ETS) and depth of response (DoR) predict overall survival (OS) in first-line chemotherapy + anti-EGFR monoclonal antibodies in KRAS wild-type metastatic colorectal cancer (mCRC). This association and the predictive accuracy of response measurements were investigated in the first-line setting for FOLFOX/FOLFIRI plus cetuximab. Methods: We performed a study of FOLFOX/FOLFIRI plus cetuximab as first-line treatment in Indian patients with KRAS wild-type mCRC. The primary endpoint was response rate (RR), and secondary endpoints included progression-free survival (PFS), overall survival (OS), and safety. Radiologic assessments at week 8 were used to calculate the relative change in the sum of the longest diameters of the target lesions. Cox regression models analysis investigated associations between ETS and overall survival (OS) and progression-free survival (PFS). Results: Sixty (78.9 %) of 76 patients had ETS, which was associated with prolonged PFS and OS. Both ETS and DoR were able to predict survival as accurately as RECIST response. Both ETS and DoR were associated with PFS and OS at the univariate analyses and in the multivariate models stratified for other prognostic variables. In the study patients, the RR, median PFS, and OS were 68.4 %, 13.1 months, and 30.6 months, respectively. Median DpR was 52%. The DpR correlated with OS as well as PFS. FOLFOX plus cetuximab was active as a first-line, with no major toxicities. Conclusions: Our prospective evaluation of chronological tumor shrinkage showed that ETS and DpR correlate with outcomes in patients with KRAS wild-type mCRC who receive cetuximab-based chemotherapy. Achieving rapid and deep tumor shrinkage consistently delays tumor progression and prolongs survival in patients treated with first-line chemotherapy plus cetuximab. ETS is a promising and valuable end point for clinical trials’ design deserving further investigation.