Nosocomial infections (NI) are important causes of morbidity and mortality in pediatric hospitals. Different surveillance methods for detecting hospital-acquired infection have been developed. The selection of which varies according to specific surveillance objectives and the resources are available. The aim of this study was to determine the epidemiology of nosocomial infections in medical wards of a tertiary-levels teaching pediatric hospital in Tehran, Iran. We performed a prospective cross-sectional study and NI was identified by daily review of medical charts of patients hospitalized for at least 48 hours. We evaluated 1497 patients. The overall patient NI rate was 3.34 per 100 patients and the infection rate per 1000 patient-days was 5.27. The most common site of infection was the respiratory tract (36%) followed by the gastrointestinal tract (32%). S. aureus, P. aeruginosa and Salmonella species were the most frequent pathogen isolates. The lethality rate associated with NI was 10%. Describing the epidemiology of NI in this hospital enable us to estimate infection occurrence, distribution and expected incidence, as well as recognizing trends and keeping track of possible outbreaks.