Objective To study MRI characteristics of viral encephalitis and the possibility of differentiating diagnosis with acute disseminated encephalomyelitis(ADEM).Methods MRI findings in 56 patients with viral encephalitis and 50 patients with ADEM were analysed and emphasized on MR findings of viral encephalitis.Results The abnormal signal lesions were found in 56 patients with viral encephalitis,including multiple or single,symmetrical or asymmetrical large patch shape lesions,the lesions located mainly in cortices,subcortical and basal ganglia and thalami.The lesions appeared as long T_1 and long T_2 on MRI.Enhanced MRI was performed in 27 cases,and demonstrated abnormal enhancement in 17 cases,including large patch or gyrus shape enhancement in 10 cases.Multiple and asymmetrical spot-patch white matter lesions were revealed in the frontal lobe,parietal lobe,occipital lobe,temporal lobe,and periventricular areas with ADEM.The lesions were long T_1 and long T_2 on MRI.Enhanced MRI was performed in 20 cases,and demonstrated abnormal enhancement in 15 cases,peripheral ring-type or patch shape enhancement was observed in all cases.Conclusion MRI is one of the important methods in diagnosis of viral encephalitis,it can provide evidences for early diagnosis and differentiation in the certain degree.