This study examined the impact of resident duty hour reform on hospital readmission rates using an observational study with interrupted time series analysis of Medicare patients admitted to hospitals from 2003 to 2010. The study found no evidence that readmission rates improved or worsened after duty hour reform when comparing hospitals of different teaching intensities. Readmission rates neither improved nor worsened in association with duty hour reform, and these findings were robust across composite measures of readmission and mortality. The results provide evidence that duty hour reform did not generally impact patient outcomes.