States have increasingly used Health Maintenance Organizations (HMOs) to provide medical services to the Medicaid population. However, the effects of these initiatives on total health-care expenses, the mix of utilization, and access to care remain unclear. We examine the effect of changes in Medicaid HMO penetration between 1996 and 2002 on these outcomes using data for the nonelderly Medicaid population in the Community Tracking Study's Household Survey. We develop market-level measures of Medicaid HMO penetration from CMS and InterStudy data, distinguish whether the HMOs specialize in serving the Medicaid population, and use a market fixed-effects model to focus on changes in HMO penetration rates over time. Although limited by imprecise estimates, we find some evidence that utilization and access are related to the market penetration rates of commercial and Medicaid-dominant HMOs, but the pattern of results we observe does not appear to be consistent with welfare improvements. Copyright © 2010 John Wiley & Sons, Ltd.