Japanese encephalitis (JE) is recognized as an important public health problem in Vietnam. A JE immunization program was introduced in 1997 in high-risk districts and expanded to additional districts over subsequent years. We reviewed national acute encephalitis syndrome (AES) surveillance data for 1998-2007 and analyzed more detailed data regarding JE from five northern provinces in 2004 and 2005. The annual reported incidence of AES in Vietnam ranged from 3.0 to 1.4 cases per 100,000 population with a decreasing trend over the 10-year period. The mean annual incidence of AES was highest in the northern region of the country. Of 421 AES cases from five northern provinces with laboratory results reported, 217 (52%) had laboratory evidence of recent JEV infection. As Vietnam moves closer to control of JE through immunization, accurate JE surveillance data will be important to evaluate and guide the program.