Background: Opportunistic infections are the most common cause of hospitalization of patients with HIV/AIDS in developing countries. This study was performed to determine the prevalence of opportunistic infections in hospitalized patients with HIV/AIDS in infectious diseases ward of Tehran Imam Khomeini Hospital (Iran) during 2009-2012.Methods: Patients with HIV/AIDS admitted to infectious diseases ward of Imam Khomeini Hospital were included in the study through census method. Gender variables, CD4 count, history of receiving antiretroviral therapy, cause of admission, length of hospitalization and patients' outcomes were recorded and analyzed.Findings: During the study, 498 patients were included in; 84.3% of whom were men. Opportunistic infections were the most common cause of hospitalization (51.2%) and the most prevalent of which were pulmonary tuberculosis (37.6%), brain toxoplasmosis (18%) and pneumocystis pneumonia (13.3%). Among the patients admitted due to opportunistic infections, 87.5% had CD4 count of less than 200 and 28.6 % had a history of receiving antiretroviral therapy. 54.3% of deceased patients had been admitted due to opportunistic infections.Conclusion: Patients suffering from opportunistic infections had significantly lower CD4 count and longer hospitalization than the others. Widespread application of antiretroviral therapy and the use of new drugs with greater efficacy as well as identification patterns of drug resistance have a dramatic importance to reduce the rate of the opportunistic infections in these patients.