Acute encephalitis syndrome (AES) is a major public health problem in eastern Uttar Pradesh, claiming thousands of lives every year. Here we report the common viral etiologic agents of AES and its epidemiology in the vicinity of Lucknow in Uttar Pradesh, North India. Cerebrospinal fluid (CSF) samples collected from patients with AES, who were referred to a viral diagnostic laboratory from January 2011 to December 2012, were tested for IgM antibodies against Japanese encephalitis virus (JEV), dengue virus (DV), herpes simplex virus (HSV), measles virus, mumps virus, varicella zoster virus (VZV), and enterovirus using commercial enzyme immuno-assays. Of the 1,578 enrolled patients, JEV was the most commonly detected (16.2%), followed by DV (10.8%), HSV (9.3%), measles virus (8.9%), mumps virus (8.7%), VZV (4.4%), and enterovirus (0%). Co-positivity with more than 1 virus was observed in 12 patients. The demographic distribution of patients pertaining to age, sex, and geographic and seasonal variation is discussed. Maximum mortality was caused by JEV infection, while patients with HSV infection had maximum residual neuro-psychiatric disability. JEV and DV are the chief causative agents of AES in North India, although other viruses should also be considered in a differential diagnosis.