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Background. Childhood bacterial meningitis is severe and largely preventable by vaccination. Few data on childhood bacterial meningitis in Northeast and Central Asia exist. Our aim was to determine the incidence and etiology of childhood bacterial meningitis in Ulaanbaatar, Mongolia. Methods. We conducted prospective, population-based, active hospital surveillance for clinical meningitis in children 2 months to 5 years of age. Clinical data, blood, and cerebrospinal fluid were collected according to a standard protocol. Laboratory testing was performed at 2 reference laboratories in Ulaanbaatar. Results. From February 2002 to January 2005, 201 suspected meningitis cases were identified in residents of Ulaanbaatar. The average annual incidence rate for confirmed and probable bacterial meningitis (cases with culture-negative, purulent cerebrospinal fluid) was 68 cases per 100,000 children aged 2 months to 5 years. The average annual incidence rate of confirmed cases was 28 cases per 100,000 children for Haemophilus influenzae type b meningitis, 11 cases per 100,000 children for pneumococcal meningitis, and 13 cases per 100,000 children for meningococcal meningitis. Adjusting for cases without complete cerebrospinal fluid information and culture-negative, probable bacterial cases, the estimated incidence rate was 40 cases per 100,000 children for H. influenzae type b meningitis, 15 cases per 100,000 children for pneumococcal meningitis, and 17 cases per 100,000 children for meningococcal meningitis. Conclusion. H. influenzae type b is the leading cause of childhood bacterial meningitis in Ulaanbaatar, and the incidence rate is higher than that reported from other Asian countries. These data supported the recent introduction of H. influenzae type b conjugate vaccine in Mongolia. Ongoing surveillance will monitor the impact of the vaccine.