Mean annual changes in shares of U.S. import markets over 1976-83 are calculated for each of eighteen less-developed countries (LDCs) exporting to the United States. Means for cases in which tariffs rose and fell under the competitive need provisions of the U.S. generalized system of preferences are compared with means for cases in which tariffs remained constant. The asymmetric results: generalized system of preferences tariff increases (decreases) reduce (do not augment) imports. Both more and less competitive LDCs lost market shares when tariffs rose on their products, but other unaffected LDCs generally did not benefit from trade diversion.