The presence of pronator drift is a sign of upper motor neurone disease.
The patient is asked to hold both arms fully extended at shoulder level in front of him, with the palms upwards.
It is a way of eliciting relatively mild spasticity. The upper motor neurone weakness may be revealed by a tendency to pronate the forearm on the affected side when the eyes are closed. This is caused by a difference in the muscular tone between pronation and supination.