Background Health care reform has expanded eligibility to public insurance without fully addressing concerns about access. We measured children’s access to outpatient specialty care to identify disparities in providers’ acceptance of Medicaid and the Children’s Health Insurance Program (CHIP) versus private insurance. Methods Between January and May 2010, research assistants called a stratified, random sample of clinics representing eight specialties in Cook County, Illinois, which has a high proportion of specialists. Callers posed as mothers of pediatric patients with common health conditions requiring outpatient specialty care. Two calls, separated by 1 month, were placed to each clinic by the same person with the use of a standardized clinical script that differed by insurance status. Results We completed 546 paired calls to 273 specialty clinics and found significant disparities in provider acceptance of Medicaid–CHIP versus private insurance across all tested specialties. Overall, 66% of Medicaid–CHIP callers (179 of 273) were denied an appointment as compared with 11% of privately insured callers (29 of 273) (relative risk, 6.2; 95% confidence interval [CI], 4.3 to 8.8; P<0.001). Among 89 clinics that accepted both insurance types, the average wait time for Medicaid–CHIP enrollees was 22 days longer than that for privately insured children (95% CI, 6.8 to 37.5; P = 0.005). Conclusions We found a disparity in access to outpatient specialty care between children with public insurance and those with private insurance. Policy interventions that encourage providers to accept patients with public insurance are needed to improve access to care.