The locked-in syndrome (LIS) is a state of an upper motor neurone quadriplegia involving the cranial nerve pairs with usually a lateral gaze palsy, paralytic mutism, full consciousness and awareness by the patient of his environment. A historical presentation of the LIS is given as well as a short description of the clinicoanatomic lesion causing LIS. The usual cause is vascular and corresponds to a pontine infarction due to an obstruction of the basilar artery but other lesions in the brainstem can also be the cause. Non-vascular aetiologies, especially traumatic, are reviewed. The use of electroencephalography (EEG), brain auditory evoked potentials (BAEP) and somesthesic evoked potentials (SEP) are discussed as well as the use in the acute stage of computed tomography (CT), angiography, and magnetic resonance imagery (MRI). The last method may show well delineated ischaemic lesions some time after the event. The communication disability is probably the most difficult to overcome. Two cases of LIS are presented.