The main purpose of this study was the exploration of the relationship between first graders' performance on traditional readiness measures and their performance on an instrument based on Piagetian tasks. All correlations among the predictive indices considered were significant (p less than .01), as were the correlations of these measures with end-of-year mathematics achievement scores. The latter ranged from .49 (teacher prediction) to .86 (Number Concept Test), and were submitted to a stepwide regression procedure. This data analysis indicated that although a regression model containing all five predictive indices was statistically significant, the most efficient model included only the Number Concept Test (74.6 percent of variance accounted for). The author recommends study of the applicability of the Piagetian items to diagnostic situations, development of Piaget-based tests for other grade levels, and the study of sources of variation on the Number Concept Test. (Author/SD)