A clinicopathologic case of acute diffuse lymphocytic meningoencephalitis in a 8-year-old child is reported. Clinicai picture started 7 days prior to death and was characterized by fever and occipital cephalea followed by worsening of general conditions, vomiting and generalized convulsive crisis together with cardiopulmonary arrest and coma. The pathologic examination showed alterations only in the brain, namely pronounced cerebral edema and inflammatory infiltrate, predominantly lymphocytic, perivascular, particularly in the white matter, in all regions analyzed. This case is compared to the ones described in the literature and the etiopathogenesis of acute diffuse lymphocytic meningoencephalitis is discussed. The necessity of making public cases of non-bacterial acute encephalitis is emphasized, due to the fact that most of them remain with indetermined etiology.