Background and purpose: The master patient’s index (MPI) card is the key to locate the patient’s record in medical records department. Use of MPI in hospital information systems is important. An accurate MPI is noted in evaluation and accreditation program. Our study was done on MPI at medical records department of teaching hospitals in Mazandaran medical university in respect of using indexing rules and arrangement of cards for filing.Materials and methods: This survey was done for inpatient cards in 5 teaching hospitals of Mazandaran medical university and 1999 MPI cards were checked. A part of data related to using indexing rules and filing was gathered with checklist and factors associated with management of department collected in questionnaire. Data processed with SPSS software and analyzed with descriptive and inference statistics.Results: Findings showed that 20% of staff responsible for indexing and filing of MPI was technician (post diploma course) in medical records. In 100% of hospitals, guide card and cross-index card wasn’t use. Indexing error increased with the increasing the number of admitted patients (p=0.003), low experience (p=0.001) and degree and no course study relation of medical records department managers (X2=9./03, p=0.000). In addition, indexing error increased in staff with no academic training (X2=3.84, p=0.05). Also results showed that there is 64.2% indexing error and 9% of cards are misfiled.Conclusion: According to the findings, staffing professional personnel, training them with up dated knowledge, periodic control of issued cards, creating needed rules and regulation based on new scientific materials, exact indexing and filling rules suggested.