Prepaid, case managed systems have been proposed as a method of controlling costs in Medicaid populations. We investigated the utilization of preventive services in two prepaid Medicaid Competition Demonstration programs in Santa Barbara County, Calif., and Jackson County, Mo. (containing the city of Kansas City). Care in the demonstration sites was compared with care given in similar counties functioning under a traditional fee-for-service Medicaid system--Ventura County, Calif., and St Louis, Mo. We tested the hypothesis that preventive care would be less in the capitated demonstrations. 2735 Children's and 3389 adult's charts were abstracted for care received during the calendar year 1985, after the prepaid demonstration had been in place for more than 1 year. No significant differences were found between the demonstration and comparison counties in the proportion of children with complete DPT or OPV immunizations at 1 year of age, with 56% complete in both California counties and 69 and 65% complete in Jackson County and St Louis, respectively. Regression analysis demonstrated a slight, but statistically significant trend towards more immunizations in the demonstration counties. Pap smear use in women of 15-44 years of age was little different in the California counties, but significantly greater in the Jackson County demonstration in Missouri (64 vs 45%). Physician breast examinations were somewhat more likely to occur in the prepaid, case managed demonstration counties. Capitated, case managed systems for the AFDC Medicaid population appear to result in no diminution of preventive services. Substantial problems exist in this, as in other poor populations, in childhood immunizations.