BACKGROUND: Patient-reported outcomes (PROs) are essential for evaluating treatment effects on health-related quality of life and symptoms from the patient’s perspective. This study sought to evaluate the psychometric properties of the nine-item Functional Assessment of Cancer Therapy/National Comprehensive Cancer Network Colorectal Cancer Symptom Index (FCSI-9) in a metastatic colorectal cancer (mCRC) population. METHODS: The FCSI-9 and EQ-5D were administered every 2-4 weeks to mCRC subjects in a phase III clinical trial. Three hundred ninety-one mCRC subjects completed the questionnaires at baseline and at least one follow-up assessment. Internal consistency reliability, test-retest reliability, construct validity, known groups validity, responsiveness, and the minimum important difference (MID) of the FCSI-9 were evaluated. RESULTS: The internal consistency and test-retest reliability of the FCSI-9 were acceptable (0.81 and 0.76, respectively). Construct validity was supported based on moderate correlations with the EQ-5D. Known groups validity was evaluated by examining the FCSI-9 scores of subjects categorized by their Eastern Cooperative Oncology Group performance status (PS) score. Subjects with better PS scores reported significantly higher FCSI-9 scores than those with lower PS scores at both baseline and week 8. Responsiveness, as measured by Guyatt’s statistic, was 0.77 from baseline to week 8 and 0.60 from week 4 to week 12. Considering all data together, the MID of the FCSI-9 is estimated to be in the range of 1.5-3.0 points. CONCLUSION: Results provide preliminary evidence of the reliability, validity, and responsiveness of the FCSI-9.