OBJECTIVE The diagnosis of maxillary sinusitis in intensive care patients remains problematic. However, it is essential since maxillary sinusitis causes numerous complications and there is an effective treatment. The aim of this study was to compare A-mode ultrasound with sinus computed tomography (CT) for the diagnosis of maxillary sinusitis in intubated patients in critical care undergoing mechanical ventilation. METHODS Prospective clinical study in 140 maxillary sinuses in the surgical ICU of a university hospital. In each intubated and mechanically ventilated patient undergoing cerebral CT scan for any reason, a bedside A-mode ultrasonography of the maxillary sinuses was performed the same day. The A-mode ultrasound result was compared with the result of the sinus CT scan for the diagnosis of maxillary sinusitis. RESULTS Sensitivity, specificity, positive predictive value and negative predictive value of A-mode ultrasound compared with CT were 66.7%, 94.7%, 75.0% and 92.2% respectively. All the empty sinuses were correctly identified as being empty. CONCLUSIONS Given its very good specificity and negative predictive value, bedside A-mode ultrasound may be a useful first-line examination for intubated and mechanically ventilated patients in intensive care, especially to eliminate suspicion of maxillary sinusitis.