Field genetic uniformity is the probability that any two plants selected at random within a region carry alleles that are identical by descent. Genetic uniformity of field crops can occur through breeding with genetically related backgrounds and grower choice of a few related cultivars among other factors. Transgenic cotton cultivars currently dominate the U.S. cottonseed market, comprising approximately 72% of the Year 2000 cotton hectares. All transgenic cotton cultivars were derived through backcross breeding with popular nontransgenic cultivars. The objective of this study was to apply pedigree analysis to estimate field genetic uniformity since transgenic cultivars were introduced into the U.S. cottonseed market in 1996. Coefficients of parentage and proportion of hectares planted to transgenic cultivars were employed to estimate field genetic uniformity for the southeastern, south-central, southwestern, and western production regions. Compared with field genetic uniformity estimated in the Year 1995 preceding introduction of transgenic cultivars, field genetic uniformity did change (0.18 vs. 0.13). The number of cultivars planted on the largest hectarage has not changed, but the percentage of the crop planted to a few cultivars has declined. The proportion of the hectarage planted to the most popular cultivar also has declined. Both of these factors affected field genetic uniformity resulting in a 28% reduction in uniformity across the USA.