This study examined differences in predicting achievement by sex on the Iowa Tests of Basic Skills (ITBS) from the verbal, quantitative, and nonverbal scores on the Cognitive Abilities Test (CogAT). The sample (n=10,000) consisted of all students in Grades 2, 5, and 8 who completed both tests in fall 1984. Examinations of means and standard deviations on CogAT showed that boys and girls had similar scores on all three CogAT areas for all grades. When ITBS scores were predicted separately from each CogAT score by sex, significant differences in either slope or intercept were found between the prediction equations for several ITBS subtests, most frequently for reading and language skills. The common regression lines for each ITBS subtest and CogAT score were computed by grade. The means and standard deviations of the residuals computed by sex showed consistent over-prediction for boys on the reading, reference materials, and language skills subtests. Girls' scores were consistently under-predicted on these subtests. These differences occurred even though average verbal, quantitative, and nonverbal ability measures were similar for boys and girls. Educators who use differences between actual ITBS achievement and expected achievement based on CogAT scores should be aware of the differences in prediction for boys and girls. (Author/PN)