SummaryBackgroundM‐mode ultrasonography might be useful for detecting hemidiaphragm paralysis. The objective of the present study was to describe the motion recorded by M‐mode ultrasonography of both diaphragmatic leaves in patients with a pre‐established diagnosis of hemidiaphragm paralysis.MethodsA study was conducted in 26 patients (18 men, 8 women) with unilateral diaphragmatic paralysis. They were referred to two different rehabilitation centres after thoracic surgery in 23 cases and cardiac interventional procedures in three cases. The pulmonary function tests and the study of the diaphragmatic motion using M‐mode ultrasonography were recorded.ResultsThe pulmonary function tests showed a restrictive pattern. The M‐mode ultrasonography reported either the absence of motion or a weak paradoxical (cranial) displacement (less than 0·5 cm) of the paralysed hemidiaphragm during quiet breathing. A paradoxical motion was recorded in all patients during voluntary sniffing, reaching around −1 cm. During deep breathing, a paradoxical motion at the beginning of the inspiration was observed. Thereafter, a re‐establishment of the motion in the craniocaudal direction was recorded. The excursions measured on the healthy side, during quiet breathing and voluntary sniffing, were increased in patients suffering from contralateral hemidiaphragm paralysis, when compared with 170 healthy volunteers.ConclusionsTo detect diaphragmatic dysfunction in patients at risk, it would be useful to study diaphragmatic motion by M‐mode ultrasonography during quiet breathing, voluntary sniffing and deep breathing.