We aimed to retrospectively measure the incidence of missed orbital blowout fracture diagnosis in primary examinations of patients with surgically treated fractures, to identify the causes of the diagnostic oversight, and to describe the clinical manifestations of the fractures. A retrospective cohort of all patients with unilateral orbital blowout fractures who underwent subsequent surgical fracture reduction at the Department of Oral and Maxillofacial Diseases, Helsinki University Hospital, from January 2011 to July 2021, was gathered. Demographics, fracture type, associated injuries, clinical manifestations, reconstruction indication, diagnostic delay, and causes of fractures were analysed. For statistical analysis, Fisher's exact test, unpaired t-test, and the Wilcoxon Rank Sum were used. Significance was set at P˂0.05. Fracture diagnosis was missed in 26 (13%) of 207 patients: in 40% of patients aged under 18 years and in 10% of patients aged 18 years or over (P=0.005). Suboptimal eye examination was found in 62% of patients with missed fracture and in 13% of those with timely diagnosis (P<0.001). Adjusted odds ratios for missed diagnosis in patients aged under 18 years versus patients aged 18 years and over was 9.3 (95% CI 2.4-35) and in patients with suboptimal versus sufficient eye examination 13.6 (95% CI 5.1-37). More common clinical manifestations in patients aged under 18 years were diplopia or restricted eye movements (P=0.005), pain in eye movements (P=0.010), nausea and/or vomiting (P<0.001), and bradycardia (P=0.014); periorbital haematoma was rarer (P<0.001). Suboptimal eye examination was involved in 62% and misinterpretation of computed tomography images in 50% of missed fractures, together explaining 85% of cases. Orbital blowout fractures are often missed in primary examination, especially in children and adolescents, who also present with subtler clinical manifestations. While the diagnosis can be difficult, appropriate clinical and radiological examination will reveal most cases.