PROJECT SUMMARY/ABSTRACT Colorectal cancer (CRC) is the second most common cause of cancer-related mortality in the United States (US). The US Preventive Services Task Force has instituted several modes of screening for early detection of CRC, such as the fecal immunochemical test (FIT) and fecal occult blood tests (FOBT). FIT and FOBT are common screening tool used in the US. FIT/FOBT must be followed by diagnostic colonoscopy when abnormal to evaluate for precancerous and cancerous colorectal lesions. The literature is scarce and has varied results on recommendations for how quickly to complete follow-up time. Studies suggesting an optimal window for diagnostic follow-up vary their recommendations from 6 months to 24 months after abnormal FIT/FOBT. Additionally, studies mainly came from international populations in Taiwan and Israel. Currently, there was only one US study examining the effect of time to diagnostic colonoscopy and colorectal outcomes. This study only looked at CRC incidence and stage of diagnosis, and not CRC mortality. Additionally, the study population was limited to the state of California. Overall, the relationship between failure to complete diagnostic colonoscopy after an abnormal FIT/FOBT on CRC outcomes has not been widely studied. Our study will use national electronic health records and claims-based data from the Department of Veterans Affairs (VA) to address key gaps in the literature by evaluating the relationship between diagnostic coloscopy and multiple CRC endpoints. First, we will examine the risk for incident and fatal CRC cancer among individuals exposed versus unexposed to diagnostic colonoscopy after abnormal FIT/FOBT (Aim 1). Next, we will determine the effect of time to colonoscopy, among individuals exposed to diagnostic colonoscopy, on CRC incidence, stage at diagnosis, and fatal cancer (Aim 2). Lastly, we will characterize the extent to which sociodemographic and clinical variables (e.g. Charlson comorbidity, age, race) impact diagnostic completion after an abnormal FIT/FOBT screening test (Aim 3).