Background An outbreak of acute encephalitis of unknown origin with high case fatality (183 of 329 cases) was reported in children from Andhra Pradesh state i n southern India during 2003. We investigated the causative agent. Methods Cell lines and peripheral blood lymphocyte co cultures were used to isolate the caus ative agent from clinical samples. Identity of the agent was established by elec tron microscopy and serological and molecular assays. Findings Clinical samples tested negative for IgM antibodies to Japanese encephalitis, West Nile, dengue, and measles viruses, and for RNA of coronavirus, paramyxovirus, enterovirus, and influenza viruses. Virus was isolated from six patients with encephalitis and w as identified as Chandipura virus by electron microscopy, complement fixation, a nd neutralisation tests. Chandipura virus RNA was detected in clinical samples f rom nine patients. Sequencing of five of these RNA samples showed 96 7-97 5% identity with the reference strain of 1965. Chandipura viral antigen and RNA wer e detected in brain tissue of a deceased child by immunofluorescent antibody tes t and PCR. Neutralising, IgG, and IgM antibodies to Chandipura virus were presen t in some patientsserum samples. Serum samples obtained after 4 days of illnes s were more frequently positive for IgM to Chandipura virus than were those obta ined earlier (p < 0 001). A similar trend was noted for neutralising antibodies . Interpretation Our findings suggest that this outbreak of acute encephalitis i n Andhra Pradesh was associated with Chandipura virus, adding to the evidence su ggesting that this virus should be considered as an important emerging pathogen. Background An outbreak of acute encephalitis of unknown origin with high case fatality (183 of 329 cases) was reported in children from Andhra Pradesh state in southern India during 2003. We investigated the causative agent. Methods Cell lines and peripheral blood lymphocyte co cultures were used to isolate the caus ative agent from clinical samples. Identity of the agent was established by elec tron ​​microscopy and serological and molecular assays. Findings Clinical samples tested negative for IgM antibodies to Japanese encephalitis, West Nile, dengue, and measles viruses, and for RNA of coronavirus, paramyxovirus, enterovirus, and influenza viruses. Virus was isolated from six patients with encephalitis and w as identified as Chandipura virus by electron microscopy, complement fixation, a nd neutralization tests. Chandipura virus RNA was detected in clinical samples f rom nine patients Sequencing of five of these RNA samples showed 96 7-97 5% identity with the reference strain of 19 65. Chandipura viral antigen and RNA wer detected in brain tissue of a deceased child by immunofluorescent antibody tes t and PCR. Neutralising, IgG, and IgM antibodies to Chandipura virus were presen t in some patients samples. Serum samples obtained after 4 days of illnes s were more frequent positive for IgM to Chandipura virus than those obtained due earlier (p <0 001). A similar trend was noted for neutralizing antibodies. Interpretation Our findings suggest that this outbreak of acute encephalitis in Andhra Pradesh was associated with Chandipura virus, adding to the evidence su ggesting that this virus should be considered as an important emerging pathogen.