Objective. To assess whether increasing enrollment in State Children's Health Insurance Programs (S-CHIPs) has an impact on the number of office physicians participating in Medicaid and the extent of their participation. Effects are measured for a freestanding S-CHIP program with an open provider panel and an S-CHIP program that uses the state's Medicaid provider panel. Data Sources. Children's Medicaid claims data for primary care services were used to measure physician participation in the program; census and enrollment data were used to describe market area characteristics. Study Design. This is a time series study of communities in two states, measuring physician Medicaid participation quarterly between 1998 and 2001, controlling for changes in community characteristics and children's program enrollment as well as other factors by quarter. Data Collection/Extraction. Office physician participation is measured by practice site. Claims data are aggregated to the level of the community and reflect the number of limited practice sites, the ratio of Medicaid office sites to the number of primary care physicians in the community as reported by the American Medical Association (AMA), and the mean number of Medicaid office visits made to physician sites in the community in the quarter. Findings. In Alabama, the state with a freestanding S-CHIP program, there is little association between increased S-CHIP enrollment and physician participation in Medicaid. In Georgia, where the same provider network serves both programs, increases in S-CHIP enrollment are associated with a decline in office-based physician participation in Medicaid in urban areas. Conclusion. Linkage of S-CHIP and Medicaid programs through the use of the same provider network, in the absence of market conditions that encourage the expansion of the network, can lead to a negative impact on access for Medicaid enrollees.