Medicaid managed care can improve access to prevention services, such as immunization, for low-income children. The authors studied immunization rates for 7,356 children on Medicaid in three managed care programs: primary care case management (PCCM; n = 4,605), a voluntary HMO program (n = 851), and a mandatory HMO program (n = 1,900). Immunization rates (3:3:1 series) in PCCM (78%) exceeded rates in the voluntary HMO program (71%), which in turn exceeded those in the mandatory HMO program (67%). Adjusting for race, urban residence, and gender, compared to children in PCCM, children in the voluntary HMO program were less likely to complete the 3:3:1 series (OR = 0.75, CI = 0.63, 0.90), and children in the mandatory HMO program were even less likely to complete the series (OR = 0.59, CI = 0.51, 0.68). Results differed by individual HMOs. Monitoring of outcomes for all types of managed care by Medicaid agencies is imperative to assure better disease prevention for low-income children.