The aim of this study was to measure the frequency of nosocomial infections in the internal medicine observation units of the emergency services in two level III hospitals in Lima. A 5-day prevalence study was carried out on patients admitted after a 72 hours observation period, in whom community-based infections were ruled out. Data was obtained from clinical records. Additionally, overcrowding and ventilation in the hospitalization rooms were evaluated. The frequency of nosocomial infections in the emergency services was found to be 8.1%, four times what was reported as period prevalence in Peru. The associated risk factors were prolonged length of stay and lack of proper ventilation in the hospital environment. Inadequate ventilation triples the risk of nosocomial infections.