We dissected 21 cadaver elbows to determine the relationship of the posterior interosseous nerve to the posterolateral approach to the elbow and head of the radius. At the distal end of the exposure the first branches at risk, those to extensor carpi ulnaris, were on average 6.0 +/- 1.0 cm (4.0 to 8.4) from the articular surface of the radial head. When using the posterolateral approach it is important that the interval between extensor carpi ulnaris and anconeus is clearly identified with the forearm fully pronated. The supinator should be released close to its ulnar border. It is safe to expose the proximal radius as far as the distal aspect of the bicipital tuberosity.