Objective: To measure key characteristics of the Veterans Health Administration's (VHA) Community Care (CC) referral network for screening colonoscopy and identify market and institutional factors associated with network size. Data Sources: VHA electronic health records, CC claim data, and National Plan and Provider Enumeration System. Study Design: In this retrospective cross-sectional study, we measure the size of the VHA's CC referral networks over time and by VHA parent facility (n = 137). We used a multivariable linear regression to identify factors associated with network size at the market-year level. Network size was measured as the number of physicians who performed at least one VHA-purchased screening colonoscopy per 1000 enrollees at baseline. Data Extraction: Data were extracted for all Veterans (n = 102,119) who underwent a screening colonoscopy purchased by the VHA from a non-VHA physician from 2018 to 2021. Principal Findings: From 2018 to 2021, median network volume of screening colonoscopies per 1000 enrollees grew from 1.6 (IQR: 0.6, 4.6) to 3.6 (IQR: 1.6, 6.6). The median network size grew from 0.63 (IQR: 0.30, 1.26) to 0.92 (IQR: 0.57, 1.63). Finally, the median procedures per physician increased from 2.5 (IQR: 1.6, 4.2) to 3.2 (IQR: 2.4, 4.7). After adjusting for baseline market characteristics, volume of screening colonoscopies was positively related to network size ([beta] = 0.15, 95% CI: [0.10, 0.20]), negatively related to procedures per physician ([beta] = -0.12, 95% CI: [-0.18, -0.05]), and positively associated with the percent of rural enrollees (0 = 0.01, 95% CI: [0.00, 0.01]). Conclusions: VHA facilities with a higher volume of VHA-purchased screening colonoscopies and more rural enrollees had more non-VHA physicians providing care. Geographic variation in referral networks may also explain differences in the effects of the MISSION Act on access to care and patient outcomes. KEYWORDS geographic factors/spatial factors/small area variations, healthcare organizations and systems, health promotion/prevention/screening, referrals and referral networks, VA healthcare system What is known on this topic * The Veterans Health Administration (VHA) has historically directly provided health care to enrolled Veterans, but recent legislation has increased its use of fee-for-service care purchased from non-VHA physicians. * Following the 2018 MISSION Act, the VHA established referral networks of non-VHA physicians who meet eligibility requirements to provide care to VHA enrollees. * The size of other referral networks, such as those for Medicaid and private insurance, has been linked to patient satisfaction, access to care, and outcomes. What this study adds * The size and volume of non-VHA physician networks for screening colonoscopies has increased since the passage of the MISSION Act in 2018. * Physicians who are active in the network perform relatively few VHA-purchased screening colonoscopies each year. * After adjusting for baseline characteristics and volume, facilities in rural areas and those with more VHA-purchased screening colonoscopies have more non-VHA physicians performing screening colonoscopies., 1 | INTRODUCTION To improve access to care for Veterans, the US Congress passed the Veterans Access, Choice, and Accountability Act (Choice Act) in 2014 and the Maintaining Systems and [...]