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. However, 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 US non-reciprocal trade preference programs, in order to shed some light on this debate. Highly disaggregated data are used to quantify the margins, coverage, utilization and value of nonagricultural and agricultural tariff preferences, for all beneficiary countries in the US regional programs and in the GSP. Results show that US 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 US tariff preferences was 5% or more of 2003 dutiable exports to the US, even after incorporating actual utilization. Most of this value is attributable to non-agricultural tariff preferences, and to apparel preferences in particular. These results suggest that preference erosion may be significant for more countries than many had thought.