Although prior knowledge is an important predictor of future performance in mathematics, few studies have analyzed which areas of the subject are most critical. Most evidence is based on small- and medium-scale studies from developed countries, with results that cannot be generalized. We explore which areas of mathematics are the most important predictors of future performance by considering the sociodemographic and academic factors of Chile. Scores from a standardized test for eighth-grade mathematics and fourth-grade language arts were analyzed for a sample of 158,818 students, together with data on different areas of fourth-grade mathematics. The results reveal that the association of future performance in mathematics with number skills is as strong as the association of future performance in mathematics with attending an elite private school, in comparison to attending a state-subsidized school. Literacy skills also proved to be an important predictor of future performance in mathematics.