Policies implemented by the Accreditation Council for Graduate Medical Education (ACGME) in 2003 limit resident's work hours to 80 hours per week, with 1 day off per week and no more than 30 consecutive hours on duty. Prior research has demonstrated that reducing the duration of work shifts can reduce serious medical errors. This prospective cohort study enrolled interns affected by the new regulations and conducted monthly surveys to assess work hours before and after implementation of work hour limitations. Although the weekly number of hours worked declined slightly, most interns reported violating the regulations during at least 1 month of their internship, most commonly by working more than 30 consecutive hours. The authors discuss factors that may account for failure to adhere to work hour regulations.