Ulnar neuropathy is a known complication of median sternotomy surgery, and its precise cause is not clear. The authors hypothesize that a "double-crush" phenomenon caused by intraoperative compression of the brachial plexus occurs in a patient with preexisting, previously undiagnosed slowing of ulnar nerve conduction. Eighty-four arms in 42 patients scheduled for cardiac surgery were studied. Pre- and postoperative history and physical examinations were performed by separate blinded examiners; however, the nerve conduction studies were performed by a single examiner. Fifteen arms (18%) in 11 patients (26%) clinically demonstrated postoperative neuropathy. After surgery, all symptomatic patients had considerable slowing of their preoperative conduction studies. A direct correlation appears to exist between preoperative slowing of ulnar nerve conduction and postmedian sternotomy neuropathy. A careful preoperative evaluation including nerve conduction studies may detect the predisposed patient; the surgeon attempting postoperative cubital tunnel decompression is cautioned that the lesion may not be an isolated one.