The stability of reading performance, as measured by the Metropolitan Achievement Tests, Iowa Tests of Basic Skills, and Iowa Tests of Educational Development, was studied using students in grades 1 through 7 and grades 9 and 11. A reading vocabulary test and a reading comprehension test are included in all three test batteries. The standard scores on the three tests were pooled to obtain a composite reading score for three independent samples of students. Sample I consisted of grades 3-6 and 9 and 11, the number of students varying from a low of 71 (grade 5) to a high of 1,116 (grade 9); Sample II was made up of students from grades 1-7 and grade 9, the number varying from 520 (grade 2) to 1,240 (grade 7); and Sample III contained students from grades 1-6, varying in number from 1,095 (grade 6) to 1,320 (grade 1). Results of the study showed that substantial long-term stability was reflected in both the vocabulary and comprehension tests; grade 1 scores correlated above .5 with all subsequent measures. By the end of the primary grades, students' scores correlated above .70 with all subsequent measures. When the coefficients were correlated for attenuation to allow an estimate of the relationships after errors of measurement on the test were removed, the values were about .10 higher. It is concluded that although reading does not represent temporary maturational status for most pupils, it does have substantial relationship with terminal achievement levels in both reading vocabulary and comprehension. (DB)