Machine-readable passports - Starting October 1, 2003, each Visa Waiver Program traveler must present a machine-readable passport (MRP) at the U.S. port of entry to enter the U.S. without a visa, otherwise a nonimmigrant visa is required. This change includes all categories of passports-- regular, diplomatic, and official, when the traveler is seeking to enter the U.S. for B-1/B-2 purposes. To learn more, see below.
Citizens of Belgium who wish to travel to the U.S. under the Visa Waiver Program must present a machine-readable passport (MRP) effective May 15, 2003.
More information can be found:
http://travel.state.gov/vwp.html