Introduction: Brachiocephalic artery rupture is a very rare and fatal condition in dogs that is likely to be missed during clinical examination and would normally be diagnosed at post mortem. The aetiology is unknown but is most likely multifactorial and associated with arterial wall degeneration leading to rupture and sudden death due to massive haemorrhage resulting in death from hypovolemic shock. Case Presentation: An intact (unspayed) Boston terrier bitch was admitted into the clinic for sudden onset lethargy. Ante-mortem differential diagnoses which included shocked lung, lung bleeding, diaphragmatic hernia, babesiosis, verminosis and internal bleeding were considered based on history, physical examination, clinical pathology and plain radiography. Conclusions: A definitive diagnosis of fatal brachiocephalic artery rupture was reached at post-mortem examination, performed pro-bono, out of interest with the owner's permission. Ante-mortem diagnosis of brachiocephalic artery rupture is still an unsolved challenge for the practising veterinarian, notwithstanding the recent advances in diagnostics. Fatal brachiocephalic rupture is a rare condition and only has been reported in dogs twice previously.