OBJECTIVE: To determine if the availability of public ambulatory clinics affects preventable hospitalization (PH) rates of low-income and elderly populations. DATA SOURCES: PH rates were calculated using elderly and low-income discharges from 1995-97 Virginia hospital discharge data. Other data sources include the 1990 Census, the 1998 Area Resource File, the 1996 American Hospital Association Survey, the Virginia Department of Health, the Virginia Primary Care Association, and the Bureau of Primary Health Care. STUDY DESIGN: Multiple linear regression was used to evaluate the relationship between ambulatory clinic availability and PH rates, controlling for population and other provider characteristics in a cross-section of zip code clusters. DATA EXTRACTION METHODS: Clusters with populations of at least 2,000 were assembled from zip codes in each county in the state of Virginia. Overlapping medical market service areas were constructed around the population centroid of each cluster. PRINCIPAL FINDINGS: Populations in medically underserved areas (MUAs) served by a Federally Qualified Health Center had significantly lower PH rates than did other MUA populations. The presence of a free clinic had a marginally significant association with lower PH rates. CONCLUSIONS: The availability of public ambulatory clinics is associated with better access to primary care among low-income and elderly populations.