The Third International Mathematics and Science Study disclosed that while U.S. students did well in the fourth grade in comparison with students from other countries, they had slipped considerably by eighth grade. This study was conducted to see what could be learned about achievement growth between grades 4 and 8. Achievement growth was investigated using the National Assessment of Educational Progress (NAEP), which has been redesigned so that it is possible to track cohorts of students and to determine the value-added in terms of education between fourth and eighth grades. When NAEP cohort records were examined, it was found that the average NAEP scores of students are slightly higher today than those of students of 20 or 25 years ago, but the same is not true of cohort growth between grades 4 and 8. Cohort growth is the same as, or lower than, it was for the earliest period for which data are available. When individual states are studied, there is little cohort growth between the fourth and eighth grades. Measuring and examining cohort growth has the potential to provide a new and important dimension in understanding trends in educational achievement. Research must then determine the factors related to cohort growth changes. (Contains 5 tables and 11 figures.) (SLD)