The State Children's Health Insurance Program (SCHIP) was implemented in 1998, providing new funds for states to cover uninsured children. This study examines the relationship between SCHIP implementation in California and hospitalizations for ambulatory care sensitive conditions (ACSCs), an indicator of primary care access and quality. We use administrative SCHIP enrollment records for urban California counties, linked with corresponding rates of hospitalization for seven ACSCs among children ages 1-18 for 1996-2000. Results from multivariate regression models indicate that increases of 1 percentage point in SCHIP enrollment are associated with reductions of 0.42 ACSC admissions per 100,000 children age 1-18 (p = 0.009). Models that use lagged effects of SCHIP enrollment indicate an even stronger relationship. These are population-level relationships, and translate to much larger effects on the specific population subset that enrolled in SCHIP. These results suggest a strong beneficial effect of SCHIP on primary care among the children covered.