Objectives: This study aimed to determine inter-observer reliability and intra-observer reproducibility of the OTA/AO classification for humeral shaft fractures, and to evaluate differences between fracture types, fracture groups, and surgical specializations. Patients of 16 years and older were included. Periprosthetic, recurrent, and pathological fractures were excluded. Radiographs were provided in random order, and observers were blinded to clinical information. To determine intra-observer agreement, radiographs were reviewed again after two months in a different random order. Agreement was assessed using kappa statistics.Results: Inter-observer agreement for the three fracture types was moderate (κ=0.60; 0.59-0.61). It was substantial for type A (κ=0.77; 0.70-0.84), and moderate for type B (κ= 0.52; 0.46-0.58) and type C fractures (κ=0.46; 0.42-0.50). Inter-observer agreement for the nine fracture groups was moderate (κ=0.48; 95% CI 0.48-0.48). Orthopedic trauma surgeons had better overall agreement for fracture types, and general orthopedic surgeons had better overall agreement for fractures groups. Observers classified 64% of fractures identically in both rounds. Intra-observer agreement was substantial for the three types (κ=0.80; 0.77-0.81) and nine groups (κ=0.80; 0.77-0.82). Intra-observer agreement showed no differences between surgical disciplines. Conclusions:The OTA/AO classification for humeral shaft fractures has a moderate interobserver and substantial intra-observer agreement for fracture types and groups.10