The pathogenesis of massive haemorrhagic transformation is not well established. Fatal haemorrhagic transformation associated with in situ dissection after acute middle cerebral artery (MCA) occlusion in a patient with atrial fibrillation is reported. An 80 year old woman with atrial fibrillation developed mild hemiparesis and aphasia. Brain CT and MRI at 4 and 5 hours, respectively, of symptom onset showed proximal MCA trunk occlusion with developing striatocapsular infarct and hypoperfusion in the superficial MCA territory. A few hours later, she developed massive bleeding into the ischaemic area and died. Pathological examination showed MCA trunk dissection, surrounded by a subarachnoid clot which communicated with the cerebral haematoma. It is suggested that direct arterial wall trauma as a result of cardioembolic MCA occlusion caused bleeding into the infarct. Secondary in situ dissection may be an overlooked mechanism of haemorrhagic transformation.