With the development of interlocking nail systems especially designed for the upper arm, standards for the operative treatment of humeral shaft fractures have appeared to change. The trumpet-like shape of the medullary cavity does not allow stable splinting with a nail alone, and therefore the bone--nail complex is commonly stabilized with interlocking bolts. Between June 1996 and June 2001, 51 fractures of the humeral shaft were treated operatively at the BG Unfallklinik Ludwigshafen with the unreamed humeral nail (UHN; Synthes). All nails were inserted by the retrograde technique. Ninety-five percent of the patients showed excellent or good shoulder function at follow-up examinations. For elbow function, 91.4% of the patients showed excellent or good results. Three out of four patients with poor elbow function had suffered from an additional injury to the brachial plexus; one patient developed heterotopic ossification. Intraoperative complications were: one iatrogenic lesion of the radial nerve, two intraoperative shaft fractures, one split at the insertion point, and one supracondylar fracture. As implants we used 7.5 mm nails in 36 cases and 6.7 mm nails in 15 cases. Among the 47 patients undergoing follow-up examinations, we found two cases of non-union. All patients were pain-free. Thirty-seven patients were very satisfied, six satisfied and four dissatisfied with the therapy. Decisive criteria for the use of a new implant are a high safety standard and simple reproducibility; these appear to be fulfilled by retrograde nailing of humeral fractures with the UHN. Interlocking nailing with the UHN enriches the range of therapeutic options for humeral shaft fractures.