BACKGROUND The retrospective study was made to evaluate the fracture patterns at the proximal humeral shaft for which the long version of a standard proximal humeral nail (PHNLV) has been used. The indication has been decided by the individual surgeons. METHODS Over a five year period 72 consecutive PHNLV cases of an acute fracture were identified and were included in the study. Mean patient age was 68.9 years. Gender ratio was m/f=22/50. 86.1% of the patients fractured their humerus by a fall, the rest by a high velocity accident. We analysed patient comorbidity, ASA score, osteoporosis, social status before accident, additional injuries affecting local soft tissues or other anatomic regions. We analysed the expansion of the fractures, dividing the humerus into five zones. Fracture morphology was categorized according to the standard AO/ASIF classification (if applicable). RESULTS Comorbidities were found in 76.4% of the patients. Almost all patients (93.1%) had been living independently at home before the accident. 47.2% of patients had osteoporosis in their medical history. Five patients (6.9%) had a primary palsy of the radial nerve. Six fractures chosen for PHNLV fixation were clearly restricted to the humeral head. The remaining 66 fractures were located in the humeral shaft (AO region 12). There were 5 segmental fractures. Of the remaining 67 fractures affecting the proximal third of the humeral shaft 49.3 percent extended into the humeral head. 98 percent of these fractures displayed spiral morphology. CONCLUSIONS Proximal humeral shaft fractures are amazingly similar to subtrochanteric and distal tibial shaft fractures: Spiral fracture types with different grades of comminution are absolutely dominant; a great proportion of the fractures extend into the humeral head with growing tendency of displacement if located closer to the humeral head. Diverging traction of deltoid and pectoralis muscle causes typical displacement if the fracture line runs in between their attachments substantiating the term 'intermuscular fracture'. A distinct classification system for proximal humeral shaft fractures seems meaningful and is proposed. CONCLUSIONS There is clear evidence of specific characteristics which differentiate proximal third humeral shaft fractures from those of midshaft and distal third. They explain the specific problems of reduction and fixation. If disrespected they will lead to higher rates of therapeutic failure.