Nonunion of the proximal humerus is a challenging problem. Since 1996, we have performed interlocking intramedullary nailing for the treatment of proximal humeral nonunions with the Straight Nail System and bone grafting. The objective of this study was to investigate the clinical outcomes of this procedure in patients with proximal humeral nonunion. We investigated 14 consecutive patients (mean age, 74.3 +/- 8.7 years). One patient was excluded because of associated brachial plexus palsy. All but 2 were initially treated conservatively. Range-of-motion exercises were started 1 week after the operation. The mean follow-up period was 37.8 months. Union was achieved in all cases without any evidence of malunion. All patients had improved range of motion of the shoulder and were satisfied with the surgical results. Mean flexion of the shoulder was 122 degrees +/- 14 degrees, and mean external rotation was 35 degrees +/- 10 degrees. Interlocking intramedullary nailing with the Straight Nail System and bone grafting offered a successful method of stable internal fixation in these complex proximal humeral nonunion cases.