Valid data are a crucial aspect of infection prevention and control programs. The aim of this study was to examine the accuracy of routine reporting in the Iranian Nosocomial Infection Surveillance System in intensive care units.A blinded retrospective review of general intensive care unit medical records was performed with a standard case-finding form. Infection control nurses (ICNs) were also interviewed to explore possible reasons for differences.The results of 951 events in 856 medical records were assessed. Sensitivity, specificity, and positive and negative predictive values of routine surveillance were 27.5%, 97.2%, 69%, and 85.3%, respectively. The results indicate 82.2%, 68.4%, 62.7%, and 57.3% under-reporting of surgical site infections, urinary tract infections, bloodstream infections, and pneumonia, respectively. Over-reporting of approximately 8%-15% was detected in 4 types of health care-associated infections (HAIs). Misinterpretation of HAI definition, high ICN workload, and inactivity of infection control link nurses were the main causes of inaccurate reporting.Under and over-reporting of HAIs are main challenges of HAIs reporting in Iran. Developing guidelines, empowering ICNs through specialized training and activating infection control link nurses are necessary to achieve more accurate data in the Iranian Nosocomial Infection Surveillance System.