The Medicare Hospital Readmissions Reduction Program (HRRP), an initiative of the Affordable Care Act, imposes considerable financial penalties on hospitals with excess thirty-day readmissions for patients with selected high-volume conditions. We investigated the intended impact of the program by examining changes in thirty-day readmissions among Medicare patients admitted for three conditions targeted by the program in New York State, compared to Medicare patients with other conditions and with privately insured patients, before and after the program’s introduction. We also examined potential unintended strategic responses by hospitals that might allow them to continue to treat target-condition patients while avoiding the readmission penalty. We found that thirty-day readmissions fell for the three conditions targeted by the HRRP, consistent with the goals of the program. Second, there also was a substantial fall in readmissions for a comparison group although not as large as for the target group, which suggests modest spillover effects in Medicare for other conditions. We did not find strong evidence of unintended effects associated with the program. These early findings suggest that the HRRP is affecting hospitals in the direction intended by the Affordable Care Act.