In recent debates, trade preference erosion has been viewed by some as damaging to developing countries, and by others as insignificant, except in a few cases. But little data have been available to back either view. The objective of this paper is to improve our measures of the size, utilization, and value of all U.S. nonreciprocal trade preference programs in order to shed light on this debate. Highly disaggregated data are used to quantify the margins, coverage, utilization, and value of agricultural and nonagricultural tariff preferences for all beneficiary countries in the U.S. regional programs and in the Generalized System of Preferences. Results show that U.S. regional tariff preference programs are generally characterized by high coverage of beneficiary countries'exports, high utilization by beneficiary countries, and low tariff preference margins (except on apparel). For 29 countries, the value of U.S. tariff preferences was 5 percent or more of 2003 dutiable exports to the United States, even after incorporating actual utilization. Most of this value is attributable to nonagricultural tariff preferences, and to apparel preferences in particular. These results suggest that preference erosion may be significant for more countries than many had thought.