A prevalence survey was carried out in the Bab El Oued Universitary Hospital involving all hospitalized patients, except those stayed less than 48 hours, to estimate the magnitude of nosocomial infections. Using defined criteria, one and only physician identified nosocomial infections among 426 patients. The result showed that 69 patients (prevalence : 16.2 %) had 82 infections. The most frequent infections were wound (35.4 % of the total) followed by urinary-tract (29.3 %), skin infection associated or no with an intravenous catheter (22.0 %) and pneumonia (12.2 %). More than the half of detected infections (56.1 %) were not active at the moment of survey. The infection rate among clean wounds was 8.6 %. Prevalences increased with increasing of duration of hospital stay. The rate which was 4.8 % for patients with duration smaller than 8 days reached 31.1 % for patients with duration higher than 31 days. Prevalences varied also according underlying pathology and investigations. Young subjects under 20 years and the age group 40-59 years had higher rates of infection. There was no significant differences in the prevalence rates between males and females for all types of infections. This study emphasized the need of surveillance and prevention of nosocomial infections. As a first stage, an infection control committee must be installed.