During the summer of 1996 an unusual clustering of meningoencephalitis cases was recorded in the Capital City, Bucharest, and in some areas from South-East Romania. After an initial suspicion of an enteroviral etiology was discarded, the West Nile etiology was confirmed by specific antibodies demonstration through hemagglutination-inhibition and ELISA tests. This study included 251 patients with the diagnoses of West Nile acute encephalitis (166 cases), acute meningitis (57 cases) and acute febrile disease (33 cases). The patients' age ranged from 1 to 89 years (mean 51.1 years). The most frequent clinical manifestations were: fever (95.7% of cases), cephalalgia (92.6%), stiffness of the neck (89.1%), vomiting (62.5%), marked asthenia (46.5%), myalgia (28.9%). In addition, patients with encephalitis exhibited: alteration of consciousness (89.2% of cases), tremor of extremities (40.4%), ataxia (44%), paralysis (15.1%). The fatality rate was 15.1% in acute encephalitis, 1.8% in acute meningitis and 0% in the acute febrile disease.