OBJECTIVE: We tried to make real time observation of the ulnar nerve with elbows in an extended and flexed state at 100 degrees. METHOD: We examined 58 elbows of 29 healthy volunteers. The participants were symptomless and showed normal conduction across the elbow. The transducer was applied between the line connecting medial epicondyle and olecranon. We measured the distances between the center of nerve, medial epicondyle, olecranon, skin, and investigated the flattening ratio with elbows extended. Afterwards, we repeated the measurement with the elbows flexed at 100 degrees. We classified the position of the nerves into three groups according to the flexed position. We used the Okamoto classification. RESULTS: The distance between nerve and skin, between nerve and medial epicondyle was 0.57+/-0.11 cm, 0.83+/-0.15 cm, with the elbow extended. But with the elbow flexed, the distance decreased to a value of 0.45+/-0.11 cm, 0.64+/-0.25 cm, respectively. The flattening ratio was 0.52+/-0.13 at extension, and 0.31+/-0.11 at flexion. Subluxation and dislocation of the ulnar nerve were seen in 20.7% and 5.2% respectively. With the elbow flexed, the ulnar nerve moved anteromedially and superficially in the dislocated group. CONCLUSION: With the elbow flexed, the ulnar nerve moves superficially and medially, and the flattening ratio is greater when the elbow is extended.