Differential item performance (DIP) is discussed as a concept that does not necessarily imply item bias or unfairness to subgroups of examinees. With curriculum-based achievement tests, DIP is presented as a valid reflection of group differences in requisite skills and instruction. Using data from a national testing of the ACT Assessment, this study investigated the hypothesis that differences in high school instruction are the cause of gender-based DIP with mathematics achievement items. Research was conducted on a random sample of 2,669 college-bound, high school seniors from the October 1983 administration of the ACT Assessment Mathematics Usage Test (ACTM). Forty-five percent were males and fifty-five percent were females. The mean ACTM scaled score for the males was about one-half of a standard deviation higher than the mean for the females. The males averaged more semesters of mathematics coursework than did the females, and a higher percentage of males reported advanced or accelerated high school math courses. The results indicated that there was a substantial gender effect that could not be explained by instructional differences at the secondary school level. Geometry and word problems tended to have the greatest negative impact on female examinees. (Author/LMO)