Objectives: To investigate the factors influencing the results of prosthetic replacement operations for proximal humerus fractures. Methods: Thirty-eight patients (20 females, 18 males; mean age 60 years; range 37-83 years) were retrospectively evaluated. Fractures were classified as Neer type IV (n=18), fracture-dislocation (n=16), type III (n=3), and type II (n=1). Mean delay for surgery was 12 days (range 2-90 days) for primary cases and 135 days (range 90-180 days) for secondary operations (n=4). Clinical outcomes were evaluated using the Neer criteria, Constant scores and elevation degrees. Radiological parameters were union of the greater tubercle and bone stock. Humeral offset, lateral humeral offset, the prosthesis-acromion distance and the position of the greater tubercle relative to the acromion and the head of the implant were measured and evaluated with respect to pain, general outcome, and elevation degree. Mean follow-up was 34 months (range 8-80 months). Results: According to the Neer criteria, the results were excellent in 12 patients (31.5%), good in 12 (31.5%), poor in 13 (34%) and poor related to limited function in one patient (%2.6). All secondary cases had poor results. Mean Constant score was 64 (range 10-98) and mean elevation was 107¡ (range 30-180¡). Thirty-two patients (84%) had no or mild pain. Patients undergoing surgery within 14 days following injury had better functional results (p=0.001) and less pain (p=0.018). No correlations were found between age, fracture type and clinical success and pain status. A significant positive correlation was found between the humeral offset and the elevation degree (p=0.002). The most common complications were those associated with the greater tubercle (52%), adversely affecting the clinical success (p=0.001). Complications related to surgical technique were observed in %24 of the primary cases. Two secondary cases required removal of the prosthesis for deep infection and lysis of the greater tubercle, respectively. Conclusion: The study showed that preoperative delay, previous surgeries, problems of the greater tubercle and the humeral offset were the parameters affecting the clinical outcome following prosthetic replacement in proximal humeral fractures.