The authors analyzed in published studies providing histologic data the value of diagnostic means of nosocomial bronchopneumonia (NBP) in critically ill patients. At least 10% of patients whose trachea is intubated and the lungs mechanically ventilated suffer an histologically-confirmed NBP. Histology of NBP consists of non systematized foci of infection, localised in 90% of cases in the posterior segments of the lower pulmonary lobes. In fact, small foci are disseminated in both lungs, usually within large areas of non infectious pulmonary lesions and are often multimicrobial. This may explain why the clinical and radiological diagnosis is inaccurate in more than 40% of cases. On chest X-ray, aeric bronchogram and alveolar infiltrates are suggestive but non-specific for NBP, as are also fever, leukocytosis and purulent sputum. Therefore, to confirm the diagnosis of infection and to isolate the microorganisms responsible for NBP, additional investigations are required whose interpretation is uneasy. In a simplified manner, bacteriological inoculum takes a course parallel to the histological bronchopulmonary lesions: no bacteria in the absence of infection, high bacterial inoculum in case of confluent NBP, intermediary in case of bronchitis, however with exceptions when antibiotherapy has recently been modified. In fact, neither bronchial protected brushing nor broncho-alveolar lavage techniques have a perfect sensitivity and specificity. As an example, a diagnostic procedure in assessed in calculating the probability of an accurate diagnosis of NBP. It is concluded that diagnosis of ventilator-associated pneumonia remains still unreliable with the means available today.