Since the publication of "A Nation at Risk," (U.S. National Commission on Excellence in Education, 1983), several state and local educational authorities have increased the number of mathematics and science courses required for high school graduation. This report examines the relationships between the numbers of courses in mathematics and science that high school students complete and their achievement on standardized tests. Data are from the second follow-up survey of the National Education Longitudinal Study of 1988. Findings include: males and females did not differ significantly in the numbers of science and mathematics courses they complete; students from higher socioeconomic-status families completed more courses in these subjects than students from lower socioeconomic-status families; Asians completed more courses in math and science than Whites, and Whites completed more courses than Blacks and Hispanics; among students with comparable socioeconomic status, the differences in the number of courses completed between Whites, Blacks, and Hispanics are insignificant; test score increases from the end of the 8th grade to the end of the 12th grade are strongly related to the number of math and science courses students completed in high school; and students who completed more math and science courses show greater achievement score gains during high school, regardless of gender, race-ethnicity, and socioeconomic status. (MKR)