Using data drawn from the 1977–1978 National Assessment of Educational Progress in mathematics, this study examined the proportion of variance in mathematics achievement attributable to differences in the number of semesters of mathematics studied after taking into account the influence of antecedent conditions, home and community environment, and previous mathematics learning. Background variables accounted for 25 percent of the variance, while exposure to mathematics courses explained an additional 34 percent of the variance. The study was replicated on three different national samples of 2,200 students each using three different measures of mathematics achievement. Similar results were found in each replication.