This study examines associations between hospitalization for ambulatory care sensitive (ACS) conditions and insurance status for working age adults, and for people age 65 and older. ACS hospitalization is a recognized indicator of access to primary care. Using data from the 1997 U.S. Nationwide Inpatient Sample and the U.S. Census, we calculate population-based rates of ACS hospitalization. We also use the 1997 Medical Expenditure Panel Survey to calculate the prevalence of ACS conditions in the groups studied. Among working age adults, those receiving Medicaid and the uninsured had higher ACS hospitalization rates than insured individuals, even after adjusting for the prevalence of ACS conditions. Among Medicare beneficiaries, those who also received Medicaid benefits had higher ACS hospitalization rates than others, again after adjusting for the prevalence of ACS conditions; those with private insurance supplementing Medicare had lower ACS hospitalization rates.