Objective To study the clinical and pathological manifestations of microsporidian encephalitis.Methods The clinical findings and the brain pathological features of a patient with microsporidian encephalitis hospitalized in 2004 were studied.Results The onset was subacute or chronic. The body temperature was usually normal or below 37.5℃,but it rose when patient's condition deteriorated and coma appeaxed.The patient had hypoimmunity but without human immunodeficiency virus infection. Multifocal lesions in the whole brain,signs of meningeal irritation and infective myelogram were observed. Rheumatoid factor increased in the early stage and indirect bilirubin,proteins in cerebrospinal fluid(CSF), and immunoglobulin IgG,IgA increased in the middle stage.Cytological examination of CSF showed lymphocyte reaction.Blood routine test showed normal eosinophil granulocyte count.The patient was found to have pleurisy,peritonitis and cystitis.Brain magnetic resonance image(MRI)manifested plaque-like isometric T1 weight image and long T2 weight image signal in white matter of bilateral cerebral hemisphere and cerebella where FLAIR sequence showed hyperintensity.No apparent mass was identified.Contrast- enhanced MRI scan showed patchy and ring-like intensification.The neural system impairments were permanent and not improved after treatment.The pathology of brain tissue showed neuronal degeneration, karyopycnosis and Derivasculitis.The infectious agents were observed in the cytoplasm of neurons.Wister rats had muhiple organ inflammatory reaction 2 weeks after intraperitoneal inoculation of the patient's CSF and a large quantity of pathogens were found in the peritoneal lavage fluid.Conclusions The patient was PAS staining method is useful for detecting the pathogen in neurons and the rate can be raised by animal intraperitoneal cultivation