In the Hospital Readmission Reduction Program (HRRP), the Centers for Medicare & Medicaid Services (CMS) utilizes a planned/unplanned algorithm to prevent hospitals from being penalized for scheduled rehospitalizations. We evaluated version 3.0 of the CMS planned readmission algorithm and hypothesized that some readmissions categorized as planned by the HRRP algorithm may actually be unplanned. We identified 143,054 index admissions and 16,116 thirty-day readmissions for 131 hospitals. Only 1252 readmissions were considered planned according to Medicare’s readmission algorithm. The majority of these planned readmissions (723 [57.8%]) had an “emergent” or “urgent” admission type listed on the readmission claim, and many (513 [41.0%]) had emergency department charges, suggesting unanticipated returns to the hospital. HRRP should consider using the admission type variable and/or the presence of emergency department charges as a source of information when determining whether a readmission is planned or unplanned.