Infection control and hospital-acquired infection (HAI) prevalence have recently attracted much attention especially in developing countries. The aim of this study was to estimate the prevalence of HAI in public hospitals in Tehran, Iran, more specifically, to determine the epidemiological status of nosocomial infections in intensive care units (ICUs) of 3 public hospitals. The study was conducted in 3 governmental hospitals and all patients who had been in the hospital for at least 48 hours and had a primary diagnosis of HAI were included in the study. The study was performed on 1470 patients who were admitted to the ICU from 2012-2014; among them, 200 cases were affected by infectious diseases. The data collected using the questionnaire included demographic characteristics, length of admission, and cause of hospital infection which were all verified by the Infection Control Center (ICC). The clinical signs and symptoms of patients were also controlled. The effects of seasonal variation on HAI were also investigated by analyzing the data obtained in various seasons. Statistical analysis was performed using SPSS software. The chi-square (X2) test was used to compare categorical variables and a 2-sided P-value of less than 0.05 was considered as significant. The effect of season on HAI prevalence was analyzed using Kruskal-Wallis Test. The prevalence of nosocomial infections was 13.65% in the study. Urinary tract (40.9%) and surgical wounds (26.1%) were the most common types of infections. Bacteriological analysis suggested that Escherichia coli (35.6%) and Staphylococcus aureus (26%) were the most prevalent microorganisms related to the incidence of HAI. The results obtained showed that HAIs were more prevalent during spring. The prevalence of nosocomial infection observed in the present study seems quite high. The results showed that the length of hospitalization was positively associated with HAI. Statistical analysis showed that long length of hospitalization, age, and season are risk factors for the development of HAI during hospitalization. Furthermore, like similar studies, gram-negative bacteria were found to be the most common cause of nosocomial infections.