WE applied the methodology of evoked potentials (EP) to reveal the functional level of abnormality in a patient with circumscribed complete anaesthesia due to conversion disorder. EP components related to sensory and perceptual processing of both innocuous electrical and noxious laser stimuli were normal. However, a P300 component indicating cognitive processing failed to appear when using a modified oddball task with rare stimuli applied to the anaesthetic right hand. P300 was present with this paradigm stimulating the healthy left hand, as well as in a ‘malingerer’ – a healthy subject who was instructed to feign the same deficit. These results suggest cognitive deficits underlying sensory loss as conversion symptom which can be differentiated from malingering by use of P300.