In this article, we report the findings of an exploratory empirical study that investigated the relationship between English Language Proficiency (ELP) on performance on the Woodcock-Johnson Tests of Cognitive Abilities-Third Edition (WJ III) when administered in English to bilingual students of varying levels of ELP. Sixty-one second-grade students, identified as Limited English Proficient, were recruited from a suburban public school district and were given the WJ III in addition to their annual state standardized assessment of ELP. The findings of this study provide evidence to support a linear, inverse relationship between ELP and performance on tests that require higher levels of English language development and mainstream cultural knowledge. The implications of the findings of the present study suggest that practitioners must consider an examinee's level of developmental language proficiency and cultural knowledge acquisition as continuous variables when determining the impact of such factors on test performance and evaluation regarding whether scores obtained from tests administered in English are indeed valid for interpretation.