A case with a median nerve lesion at the wrist without thenar atrophy and another with an ulnar nerve lesion at the elbow sparing the first dorsal interosseous are reported. Simultaneous multiple channel recording demonstrated in the first case a dual innervation of the abductor pollicis brevis by median and ulnar fibers via a Martin-Gruber anastomosis. In the second case, the cross over of the median fibers innervating the first dorsal interosseous was very proximal in the antecubital fossa. Stimulation at discrete points documented that the communicating fibers from the median joined the ulnar nerve 3 cm below the medial epicondyle.