In the laboratory an ulnar osteotomy was performed in 10 cadaveric specimens, which included 5 distal-third junctions and 5 middle-third junctions. The interosseous membrane was left intact. Each specimen was rotated through full pronation and full supination. Displacement less than half shaft width was considered stable. The interosseous membrane was subsequently released 2 cm proximal and 2 cm distal to the osteotomy. Repeated pronation and supination range of motion documented gross displacement greater than half of the width of the shaft and was diagnosed as unstable. To evaluate this hypothesis, a clinical trial of 30 stable ulnar shaft fractures of the middle and distal thirds were treated with functional bracing between 1984 and 1990. A simple protocol was followed: A long arm cast was applied for approximately 5-7 days until the patient experienced only mild pain. At 5-7 days, a prefabricated forearm brace was fitted and the patient began physical therapy, advancing the upper extremity to full functional use. The bracing was continued until radiographic union had occurred. Twenty-nine clinical cases were evaluated to fracture union. The mean time to union was 7.3 weeks (range: 6-9 weeks). One delayed union was reported in the series. At 16 weeks surgical intervention was recommended and an open reduction and internal fixation with cancellous bone grafting was performed. Twenty-nine patients regained full range of motion, and one had limited supination/pronation due to a previous injury of the radial head. In conclusion, functional bracing of stable ulnar shaft fractures leads to a high incidence of fracture union and achieves good to excellent functional results.(ABSTRACT TRUNCATED AT 250 WORDS)