We describe five cases of high signal in the cerebrospinal fluid (CSF) on fast-FLAIR images 24-48 h after onset of stroke. All the patients had undergone perfusion-weighted MRI within 6 h of the onset of the symptoms. The CSF was far brighter than the cortical gyri. The high signal was diffusely around both cerebral hemispheres in two cases and around one hemisphere in two others; it was focal, around the acute ischaemic lesion, in one. CT was normal in all cases. The CSF high signal was transient, decreasing in extent and intensity with time and resolving completely within 3-6 days. It was not associated with worsening of the clinical state or poor outcome. Our explanation of this phenomena is hypothetical: we speculate that it could be due to disruption of the blood-brain barrier resulting in leakage of protein, gadolinium chelates, or both in to the subarachnoid space. It should not be confused with subarachnoid haemorrhage.