Background and Objectives: Antimicrobial resistance (AMR) is an increasing threat for efficient treatment of infections. Determining the epidemiology of healthcare-associated infections and causative agents in various hospital wards helps appropriate selection of antimicrobial agents. Materials and Methods: This retrospective study was performed by analyzing antibiograms from March 2017 to March 2018 among patients admitted to the different wards of Imam Khomeini Hospital Complex in Tehran, Iran. Results: Among 2440 hospital acquired infections, 59.3% were Gram-negative bacilli: E. coli (n = 469, 22.2%), K. pneumoniae (n = 457, 21.7%), Acinetobacter spp. (n = 282, 13.4%), P. aeruginosa (n = 139, 6.6%) and important Gram-positive bacteria were Enterococcus spp. (n = 216, 10.2%), S. aureus (n = 148, 7%), S. epidermidis (n = 118, 5.6). Generally, there was a high antimicrobial resistance in bacterial isolates in this study. Methicillin resistant Staphylococcus aureus (MRSA) was 56.3 % and MRSE 62.9 %. Vancomycin resistant enterococci (VRE) was 60.7%. K. pneumoniae- ESBL was 79.6% and its resistance to carbapenem was 38.4%. E. coli-ESBL was 42% and its resistance to carbapenems was 2.3%. P. aeruginosa resistance to ceftazidime was 74.4%, to fluroquinolones 63.3%, to aminoglycosides 64.8%, to piperacillin tazobactam 47.6% and to carbapenems 62.1%. Acinetobacter baumannii resistance to ceftazidime was 98.7%, to fluroquinolones 97%, to aminoglycosides 95.9%, to ampicillin sulbactam 84%, to carbapenems 96.4% and to colistin 4%. Conclusion: The study revealed an alarming rate of resistance to the commonly used antimicrobial agents used in treating HAIs. Also the relationship between AMR and some risk factors and thus taking steps towards controlling them have been shown.