OBJECTIVES: Bacterial meningitis is associated with high mortality and long-term complications. This study assessed the impact of Haemophilus influenzae type b (Hib) conjugate vaccine on childhood bacterial meningitis in Ulaanbaatar, Mongolia. STUDY DESIGN: Prospective, active, population-based surveillance for suspected meningitis in children aged 2-59 months was conducted (February 2002-January 2011) in 6 hospitals. Clinical data, blood, and cerebrospinal fluid were collected. The impact of Hib conjugate vaccine was assessed by comparing Hib and all cause meningitis data in the 3 years preceding pentavalent conjugate vaccine implementation (2002-2004) with 3 years postimplementation (2008-2010). RESULTS: Five hundred eleven cases of suspected meningitis were identified from 2002-2011. Pentavalent conjugate vaccine coverage in December 2005 in Ulaanbaatar city was 97%. The proportion of suspected cases confirmed as Hib meningitis decreased from 25% (50/201) in the prevaccination era to 2% (4/193) in the postvaccination era (P < .0001). The annual incidence of Hib decreased from 28 cases per 100,000 children in 2002-2005 to 2 per 100,000 in 2008-2010 (P < .0001). CONCLUSIONS: This article demonstrates the marked impact of Hib conjugate vaccine introduction on meningitis in Mongolia. It is important to sustain this surveillance system to monitor the long-term impact of Hib conjugate vaccine, as well as other interventions such as pneumococcal and meningococcal vaccines.