BACKGROUND: Humeral shaft fracture non-union rates of up to 33% have been reported when managed non-operatively. When managed surgically, non-union rates of 10% have been seen. The initial radiographic displacement parameters may be of significance in determining whether non-operative treatment might fail. AIMS: To determine whether the initial radiographic displacement of humeral shaft fractures can predict non-union rates and assist in decision-making for surgical intervention. METHODS: A retrospective cohort study was performed to identify all patients with humeral shaft fractures presenting to a regional trauma centre over a three-year period. Three-observer measurements were taken on defined radiographic parameters. These were compared between groups treated successfully with surgery and with non-operative intervention. A subset group was identified who failed non-operative treatment and required delayed surgery. Statistical analysis was performed to determine whether the group that failed non-operative treatment met the proposed radiographic parameters predicting treatment failure. RESULTS: Eighty patients were identified over the defined three-year period. Failure of non-operative management occurred in 6/43 (13.95%) patients. Failed conservative treatment was associated with increased age, female gender, and increased AP translation of lateral radiographs. Fractures successfully treated non-operatively showed a significantly lower AP translation on the lateral radiographs compared to patients who failed non-operative treatment 9.69 mm (IQR 4.90-14.05 mm) versus 22.61 mm (IQR 15.73-23.83 mm), p-value = 0.042. CONCLUSION: Significant initial AP displacement may be associated with failure of non-operative management. This study highlights the importance of initial radiographic parameters of displacement in predicting possible failure of non-operative management for midshaft humerus fractures.