OBJECTIVE To compare B-mode ultrasound with sinus computed tomograph (CT) scan in the diagnosis of sinusitis in intubated patients undergoing mechanical ventilation. METHODS Prospective, clinical investigation. METHODS Medical intensive care unit of a university hospital. METHODS Fifty patients undergoing intubation and mechanical ventilation more than 2 days, with a clinical suspicion of paranasal sinusitis with purulent nasal discharge. METHODS One hundred paranasal sinuses were examined. A paranasal CT scan and a B-mode ultrasound were performed the same day. Radiologic maxillary sinusitis (RMS) was defined as complete opacification of the sinus or as the presence of an air-fluid level. Absence of RMS was defined as normal sinus or as the presence of mucosal thickening. Important RMS was defined by total opacity or air-fluid level larger than half of the sinus area. Moderate RMS was defined by air-fluid level inferior than half of the sinus area. For ultrasonographic procedure, the image defined as normal was an acoustic shadow arising from the front wall. Two levels of positive echography were described: 1) a moderate lesion was defined as the visualization only of the hyperechogenic posterior wall of the sinus; 2) an important lesion was defined as the hyperechogenic visualization of posterior wall and the extension by the internal wall of the sinus outlining the hypoechogenic sinus cavity. RESULTS Sensibility, specificity, positive predictive value, and negative predictive value of B-mode ultrasound compared with CT were, respectively: 100% (95% confidence intervals [95% CI] = 94.9-100.0), 96.7% (95% CI = 82.8-99.9), 98.6% (95% CI = 92.4-99.9), and 100% (95% CI = 88.1-100). The concordance between a moderate B-mode ultrasound lesion and a moderate RMS on CT, and between an important B-mode ultrasound lesion and an important RMS on CT, assessed using kappa statistics was 93%. The concordance between B-mode ultrasound's results and CT's results assessed using weighted kappa statistics was 97%. CONCLUSIONS B-mode ultrasound may be proposed first-line in a ventilated patient with suspicion of maxillary sinusitis.