OBJECTIVES: Determine the risk factors and germs responsible for early-onset (E) and late-onset (L) nosocomial broncho-pulmonary infections (NBPI), in order to improve preventive strategies and the choice of initial antibiotherapy. METHODS: An observational prospective study conducted in an intensive care unit of 30 beds, from March 1993 to September 1999. The patients presenting with an ENBPI and those with an LNBPI were compared with patients without NBPI using univariate and then multivariate analysis. RESULTS: 517 (14%) of early-onset NBPI were diagnosed, but the majority of NBPI were late-onset (87%). Multiresistant bacteria predominated. The similarity in the germs responsible for the early and late onset forms of NBPI was probably related to the large number of patients transferred from other departments (82%) and having already received antibiotics before their admission to the intensive care unit (49%). Multivariate analysis identified anti-ulcer and long term corticosteroid treatments as common risk factors for early and late onset forms of NBPI, digestive failure, tracheotomy and kidney failure as risk factors for ENBPI and the number of antibiotics used in intensive care and the duration of mechanical ventilation as factors of risk for LNBPI. CONCLUSION: The limited use of antibiotics and anti-ulcer agents could improve the prevention of early and late onset forms of NBPI. The distinction in intensive care between the two forms of NBPI must be emphasized by the notion of prior hospitalization.