In 2003, U.S. students' mathematics performance was assessed by the Trends in International Mathematics and Science Study (TIMSS) for students in grades 4 and 8 and by the Program for International Student Assessment (PISA) for students at age 15. Results in the press and later summaries, such as that found in The Condition of Education (NCES, 2005b) reported U.S. rankings in relation to all participating countries on each assessment. Because of the variability in the composition of countries participating in each assessment, these discussions have given an inaccurate impression that U.S. students' performance on PISA experienced a precipitous decline compared with favorable U.S. rankings on TIMSS at grades 4 and 8. A total of 24 countries participated in TIMSS-4, 45 countries in TIMSS-8, and 40 countries in PISA. Notably, many higher-performing European countries that participated in PISA and contributed to the lower U.S. rankings were absent from the TIMSS results in which U.S. performance ranked above average. Reform proposals, such as those from the National Academy of Science and the Business Roundtable, have focused on improving mathematics at the secondary school level and may have been mislead by the differing country comparisons. The current study reexamines the TIMSS and PISA results to correct this comparison bias by analyzing U.S. mathematics performance relative to a common set of 12 countries that participated in all three assessments. Along with the United States, the comparison countries are Australia, Belgium, Hong Kong, Hungary, Italy, Japan, Latvia, Netherlands, New Zealand, Norway, and the Russian Federation. These countries span the four continents of Australia/Oceana, Asia, North America and Europe and constitute a broad range of primarily industrialized nations (i.e., above the world average per capita income). This study first examines the results for the United States compared with the results for the comparison countries. The country results are then used to explore various educational factors associated with international mathematics performance at different stages of students' mathematical development and through different country-specific characteristics. (Contains 9 exhibits and 17 endnotes.)