BACKGROUND: To determine whether the change in tumor diameters at the first follow-up computed tomography (CT) examination after baseline examination (first change) correlates with outcome in patients with metastatic colorectal cancer (mCRC) treated with combination chemotherapy. PATIENTS AND METHODS: The first change was analyzed in a multicenter randomized phase III trial (Nordic VI, N = 567) comparing first-line irinotecan with either bolus or infused 5-fluorouracil. Cox proportional hazards multiple regression model and Kaplan-Meier survival analyses after correction for guarantee-time bias were carried out to evaluate correlations between first change, objective response according to RECIST 1.0, progression-free survival (PFS), and overall survival (OS). RESULTS: The hazard ratios for PFS and OS decreased along with first change. A decrease between 10% and 20%. CONCLUSIONS: The change in tumor size at the first follow-up CT is strongly prognostic for PFS and OS in mCRC.