Data from four different school sites nationwide were analyzed to examine whether standardized test results may be confounded by the lack of language proficiency of English language learners. Several analyses comparing the performance of limited English proficient (LEP) students and their non-LEP classmates revealed major differences. A Disparity Index was created to measure the performance gap between LEP and non-LEP students on tests with varying levels of language demand. The more linguistically complex the nature of the test, the greater was the Disparity Index of non-LEP students' results over LEP students'. This may suggest that high-language-load test items in assessments of content such as mathematics and science may act as a sources of measurement error. LEP students tended to have lower internal consistency scores on standardized assessments. Again, this suggests that item language load may interfere with testing the intended constructs. Using multiple regression, multivariate analysis of variance, and canonical correlation, researchers found that the more language load in a test, the stronger the confounding between LEP status and content-based performance on that test. Structural models for LEP student results demonstrated a lower statistical fit among test items, as well as between items and the total test scores. The factor loadings were generally lower for LEP students, and the correlations between the latent content-based variables were weaker as well. (Contains 36 figures, 106 tables, and 23 references.) (SLD)