486 Background: The purpose of this study was to investigate the potential of circulating tumor cells (CTCs) as a surrogate marker of clinical outcome in metastatic colorectal cancer (mCRC) patients in order to identify Japanese patients responsive to oxaliplatin-based chemotherapy. Methods: The treatment regimen was oxaliplatin-based chemotherapy. Collection of CTCs from whole blood was performed at baseline and at 2 and 8-12 weeks after initiation of chemotherapy. Isolation and enumeration of CTCs was performed using immunomagnetics. Results: Between January 2007 and April 2008, 64 patients with mCRC were enrolled in this prospective study.Patients with ≥3 CTCs at baseline and at 2 and 8-12 weeks had a shorter median progression-free survival (8.5, 7.3, and 1.9 months, respectively) than those with <3 CTCs (9.7, 10.4 and 9.1 months, respectively) (log-rank test: p=0.047, p<0.001, and p<0.001, respectively). Patients with ≥3 CTCs at 2 and 8-12 weeks had a shorter median overall survival (10.2 and 4.1 months, respectively) than those with <3 CTCs (29.1 and 29.1 months, respectively) (p<0.001 and p=0.001, respectively). A spurious early rise in CEA level was observed in 11 patients showing a partial response. On the other hand, no rise in early CTC level was observed among responders. Conclusions: The clinical utility of CTC enumeration in improving our ability to accurately assess treatment benefit and in expediting the identification of effective treatment regimens for individual Japanese patients.