Background: An observational cohort study plays a crucial role to understand the current status of clinical practice and can be utilized as database for multi-purpose outcome research. Such database is available in Europe and the United States based on several cohort studies especially in mCRC, while there is no database available including treatments for mCRC patients in Japan. Methods: We planned and conducted a large cohort study to establish database available including treatments for mCRC patients. Major enrollment criteria included documented mCRC and first line CT containing fluorouracil, oxaliplatin and bevacizumab, started treatment in or after January 2010. The primary objectives are to evaluate overall survival(OS), liver metastasectomy rate (LMR), R0 liver resection rate (R0). The secondary objectives are progression-free survival, response rate (RR), and safety. We performed a preplanned interim analysis of 6-month efficacy and safety data after the 500th patient registration. Results: From October 2010 to September 2011, data from 1,353 patients were recruited from 132 centers in Japan, and we analyzed data on 538 patients of them. The background of 538 patients were as the following; male/female, 335/203; median age of 65 (range, 27-85); ECOG PS 0/1/2/3, 453/77/6/2; site of primary tumor colon/rectum/colorectal, 299/238/1; and site of metastatic disease liver/lung/others, 352/162/226; FOLFOX/ CapeOx/ others with bevacizumab, 234/290/14, KRAS-status wild/mutant/unknown, 131/88/319. 6-month efficacy and safety data were OS 97.5%, LMR 15.1%, R0 12.5%, RR 48.3%, G3/4(CTC-AEv4.0) related to bevacizumab; hypertension 1.5%, proteinuria 0.2%, thromboembolism 0.6%, bleeding 1.0%, and gastrointestinal perforation 1.3%. Conclusions: We have started the large Japanese cohort study which investigates first line CT for mCRC. We performed a preplanned interim analysis on 538 patients and there was no difference from past reports. We will further investigate and analyze 2-year efficacy and safety data on all 1,353 patients. This study is sponsored by the Public Health Research Center Foundation CSPOR in Japan. Clinical trial information: UMIN000006392.