We have examined the effects of permanent middle cerebral artery occlusion (pMCAO) in marmoset monkeys over 5 months, using behavioural and magnetic resonance imaging (MRI) techniques. Three marmosets were trained on behavioural tests before pMCAO. Shortly after surgery, these marmosets were scanned with T2-weighted (T2W) and diffusion-weighted (DW) MRI. Three, 10 and 20 weeks after surgery, these marmosets were re-tested on the behavioural tasks and had further MRI sessions to monitor lesion development. This was followed by histological analysis. All these marmosets had a persistent contralesional motor deficit and a spatial neglect which resolved over the 20 weeks of testing. Percentage infarct volume assessed by MRI on the day of surgery and at 20 weeks matched the percentage infarct volume measured histologically at 20 weeks. However, the apparent infarct size at 3 weeks was considerably less than that measured by histological analysis or that measured at the other MRI time points. Additional histological analysis of the brains of two further marmosets removed 3 weeks after pMCAO found considerable infiltration by lipid filled macrophages into the ischaemic zone which may have caused an MRI "fogging" effect leading to an apparent reduction in infarct volume.