The potential use of quantitative MRI to characterize early, as well as late, immune-mediated lesions of multiple sclerosis (MS) has been investigated. Experimental allergic encephalomyelitis (EAE) was induced in 4 male Macaca fascicularis monkeys, and the development of the disease followed using quantitative MRI at 0.15 Tesla. Serial scans were recorded daily, beginning at day 9 after inoculation. Lesions were detected before the onset of clinical signs, due to an elevation in the spin-lattice (T1) and spin-spin (T2) relaxation times. The T1 and T2 values from the lesions were shown to increase over time, indicative of progressive change at a molecular level. The appearance of any new lesions and any changes in existing ones were noted; this allowed dating of the lesions postmortem. Pathological correlation showed the long T1 and T2 values to be associated with the presence of inflammation, demyelination and haemorrhagic necrosis. Microscopically similar lesions had the same MRI characteristics. In addition, these studies showed the oldest lesions to be the most haemorrhagic; this is contrary to the belief that haemorrhage is a secondary event in EAE. The results show that quantitative MRI is sensitive to variable pathology, and has the potential for use in characterizing the pathological progression in MS.