INTRODUCTION: The ability to conduct community-level asthma surveillance is increasingly crucial for public health programming and child health advocacy. We explored the potential and limitations of health care use records from both public and private sources for asthma surveillance in a California county. METHODS: We combined administrative patient record data from Kaiser Permanente of Northern California and Medi-Cal (the California Medicaid program) for Alameda County residents during 2001. We assessed the resulting data set for completeness, population representation, consistency with external data, and internal indicator consistency. RESULTS: Our resulting data set included records for 226,383 children younger than 18 years. Completeness of Medicaid data was affected by managed care market share, reducing our usable data set size to 176,789, approximately equal to one of every two children in the county or one of every 3 person-months. External data documenting hospitalization rates due to asthma were poorly correlated with hospitalization rates (r = 0.2120, P = .20) but highly correlated with emergency department visits (r = 0.8607, P <.001) in the resulting data set. High internal consistency of indicators suggested that the data set represented a broad spectrum of health care access and quality of care congruent with clinical aspects of the disease. CONCLUSION: The utility of these data is affected by logistical and administrative factors, including the health care payment structure and the market shares of care providers. These factors can be expected to similarly affect the utility of this approach in other counties. Our ability to generate county-level health statistics for comparison with other locations was limited, although the data set appeared well suited for within-county geographic analysis. In light of these findings, these data have the potential to expand the local health surveillance capacity of communities.