In some intensive care, nowadays, ultrasound diagnostics have become an extension of the physical examination (like a stethoscope). In this report, we discuss the case of an acute respiratory failure which arose immediately after the end of general anesthesia. An initial bedside ultrasound evaluation applying the 'BLUE protocol' showed no pathological changes capable of explaining the clinical picture; however, by evaluating also the right and left hemidiaphragms, we made a diagnosis of diaphragmatic dysfunction, which would probably have been difficult to diagnose without the aid of the diaphragm ultrasound. We therefore decided to avoid intubation, transfer the patient to the intensive care unit, and treat him conservatively with non-invasive ventilation only. To our knowledge, this is the first case report that has shown the usefulness of ultrasonography in detecting diaphragmatic dysfunction as a cause of acute respiratory failure with a subsequent change in patient management. The use of bedside ultrasonography provides practical functional information on the diaphragmatic function in patients with acute respiratory failure and can also be easily repeated if follow-up is required. This feature is still held in little consideration, but it can affect the diagnosis and the treatment of critically ill patients.