Background: A minority of proximal humeral fractures extend into the diaphysis and their optimal treatment remains controversial. We evaluated the outcomes and risk of complications in patients with these injuries, treated by a protocol of open reduction and long locking plate fixation (LPF). Methods: Between 2007 and 2014, all locally resident patients with a proximal humeral fracture extending into the diaphysis were referred to a specialist shoulder clinic. Operative treatment using a protocol of open reduction and LPF was offered to medically fit patients. Those with 2-year radiographic follow-up were included in the study, and standardized assessments of clinical and radiographic outcomes were performed during the first 2 years postoperatively. At a mean of 8.8 years (range, 5 to 12 years) after LPF, the functional outcomes and satisfaction of surviving, cognitively intact patients were assessed with a questionnaire study. Results: One hundred and two patients met the inclusion criteria; the majority were older women who had incurred the injury during a simple fall. Fractures were divided into 2 types depending on the pattern of diaphyseal extension. The pain levels, functional scores, and satisfaction with treatment were satisfactory both at the 2-year follow-up and at the longer-term follow-up at a mean of 8.8 years postsurgery. Complications were predominantly due to postsurgical stiffness (in 7 patients, with 3 undergoing additional surgery) and nonunion or fixation failure (in 7 patients, with 6 undergoing additional surgery). Conclusions: Proximal humeral fractures with diaphyseal extension are rare. The results of our study support the use of LPF in medically stable patients in centers with the expertise to perform these procedures. Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.