OBJECTIVE This cohort study shows the results after treatment of comminuted radial head fractures combined with elbow instability, using radial head arthroplasty. METHODS Between 2001-2003, 25 patients with an unreconstructible comminuted fracture of the radial head and elbow instability were treated with a bipolar radial head prosthesis. Using the Mason/Johnston classification there were 15 type III and 10 type IV radial head fractures. In 17 cases the operation took place within a week. In 8 cases the prosthesis was implanted after 129 +/- 113 days. The mean follow-up time was 21 +/- 6 months. RESULTS The range of elbow movement showed a loss of mean extention of 13 +/- 8 degrees and a mean flexion of 126 +/- 12 degrees . The mean pronation was 72 +/- 13 degrees and supination 76 +/- 12 degrees . According to the Morrey score there were 8 excellent results, 13 good, 3 fair and 1 poor. The mean DASH score was 21.3 (0-74). Compared to a reference group, similar in age and gender, the patients showed no notable differences in quality of life. In one case the prosthesis dislocated and another patient experienced an aseptic loosening of the stem. Seven patients were diagnosed with heterotopic ossification, stage I and II on the Ilahi scale. 96 % of patients were able to return to their preoperative working life. A comparison of acute and chronic radial head fractures, treated with prosthesis showed no difference in results. CONCLUSIONS An arthroplasty with a radial head prosthesis can be recommended for unreconstructible comminuted radial head fractures with ligament injury.