Retrospectively, we reviewed the treatment and outcome of 46 isolated fractures of the ulnar shaft. Of 18 fractures treated by immediate open reduction and internal fixation, one open fracture became infected and failed to unite. Of 28 fractures treated closed, seven failed to unite. The factors prognostic of nonunion in closed treated fractures were (1) fracture in the proximal third of the ulna and (2) displacement 5 mm or more. In such cases, we recommend internal fixation.