Background: The hospitals are the most significant providers of health care services. According to more health funds spending by hospitals, economic management in hospitals is necessary. Determining the causes and rates of insurance under reimbursement bills was the objective of this study.

Materials and Methods: This research is a descriptive cross-sectional survey was done in 2008. The samples of study were the al records' patients who undertaken of social security insurance system in the teaching hospitals of Shahid Sadoughi University of Medical Science. The data complied in SPSS 16 software and then analyzed.

Results: In this study, 9.8% cases of all hospitalization bills were deducted. The highest rates of under reimbursement bills was in Afshar hospital (12.7%), followed by burn hospital (12.5% ), Shahid Sadoughi hospital (10%), and Shahid Rahnemoon hospital (8.2%). The highest rates of under reimbursement bills imposed to the surgeon fees (28.4%), treatment fees (15.06%), and medicine (14.3%) costs. However assistant surgeon fees had no under reimbursement. The most important causes of under reimbursement were defects in the insurance records of patients and noncompliance with insurance organization regulations.

Conclusion:  Training of human resources and optimized use of hospital information systems could be helpful to hospitals for reducing insurance reimbursement to ultimately improve hospitals to financial statements.