Six patients with non-traumatic anterior interosseous nerve palsy are described. In five patients the onset was acute with upper arm pain, which peaked within 4 weeks and thereafter declined. Virtually complete spontaneous recovery occurred in all patients between 9 and 24 months. It is concluded that non-traumatic anterior interosseous nerve lesions are most likely to reflect a circumscribed form of brachial neuritis and that surgical decompression should be deferred for at least a year or indefinitely if recovery is proceeding.