La Crosse encephalitis (LE), a mosquito-borne viral disease which can be mistaken for herpes simplex encephalitis, is under-recognized in the US, despite case reports from 28 states and an incidence in endemic areas(20-30/100,000) exceeding that of bacterial meningitis. The following is our 10-year experience (1987-1996) with 128 LE admissions, which is the largest single series reported and the first including experience with intracranial pressure (ICP) monitoring and specific antiviral therapy. Diagnosis was by La Crosse IgM and IgG serum titers. Typical cases occurred in school-age children(mean = 7.81, range 0.5-16 years), presenting between early July and late October. Symptoms usually involved headache and fever (83%) for 3-4 days and vomiting (71%) for 1-2 days, with progression to seizures (46%) and/or disorientation (44%). Only 24.5% had nuchal rigidity. Of those presenting with seizures, 1 in 4 occurred abruptly, and 1 in 4 evolved to status epilepticus(SE). Cerebrospinal fluid yielded: 1) white blood cells (WBC) modestly elevated (mean = 129, range 2-867), with 10% of patients having 210 WBC's; 2) negative viral cultures. Peripheral WBC's were elevated (mean = 15.7K, s.d. = 5.9K) with left shift. One in four children required intubation for seizures and/or coma and more than one-half (57.1%) required intensive care. Notably, 3 patients developed acute uncal herniation within 48 hours of admission and received IV ribavirin per FDA approval for emergency use. Those who had significant in-hospital deterioration (e.g., herniation, SE) tended to have decline in sodium (p <0.025) concurrent with the event. Except for the sentinel case in 1987, all patients have survived. LE should be considered in the the child presenting with meningoencephalitis in summer/early fall, particularly for children living in (or recent travel to) endemic areas in midatlantic and midwestern states. Early diagnosis can be made via IgM and IgG serology using indirect immunoflourescence. We are conducting a randomized clinical trial of IV ribavirin for severe cases.