The purpose of this study was to examine patterns with regard to six advanced high school mathematics courses 12th-grade students who took the American College Testing (ACT) assessment had taken between 1987 and 1997. Differences in performance on the ACT mathematics test were studied with respect to mathematics courses taken, gender, race/ethnicity, and family income. Also studied was how mathematics achievement varied by the number of mathematics courses taken. Five separate 10% systematic samples of ACT-tested 12th graders were selected in graduating classes as follows: (1) 1987, 50,779 students; (2) 1990, 52,076 students; (3) 1993, 56,749 students; (4) 1996, 57,775 students; and (5) 1997, 61,610 students. The evidence suggests that, overall, college-bound 12th graders are increasing mathematics course taking, and that the more mathematics courses students take, the higher their ACT mathematics scores. Students with higher course-taking averages tend to have higher ACT mathematics scores. Not all groups are increasing their mathematics course-taking at the same rate. Females, Blacks, Mexican Americans, and American Indian/Alaskan Natives made greater gains than others between 1987 and 1996. Students from families in the lower income level made some gains in the number of advanced mathematics courses taken, but such gains were more modest than the gains of those in the middle and higher groups. Mathematics course-taking accounted for most of the explained variance in students' achievement on the ACT mathematics test, regardless of the subgroup studied. (Contains 8 tables and 11 references.) (SLD)