Although it has been proposed that the ability to compare numerical magnitudes is related to mathematics achievement, it is not clear whether this ability "predicts" individual differences in later mathematics achievement. The current study addressed this question in typically developing children by means of a longitudinal design that examined the relationship between a number comparison task assessed at the start of formal schooling (mean age=6 years 4 months) and a general mathematics achievement test administered 1 year later. Our findings provide longitudinal evidence that the size of the individual's distance effect, calculated on the basis of reaction times, was "predictively" related to mathematics achievement. Regression analyses showed that this association was independent of age, intellectual ability, and speed of number identification. (Contains 5 tables and 1 figure.)