The Office of Civil Rights (2001) and the No Child Left Behind Act of 2001 (Antunez, 2003) mandate that language proficiency and academic achievement be measured in order to provide equal educational opportunities to English language learners and have an accountability system for their language and academic growth. The Cognitive Academic Language Proficiency (CALP) construct is often cited in the literature as a milestone to second-language (L2) development and as having a significant relationship with academic achievement in the L2. Studies have typically explored the relationship between the primary language (L1) and the L2 separately with academic achievement. Language proficiency has often been viewed as a unitary construct without considering the interrelationship between L1 and L2 (Cummins, 2001). This study investigated the cross linguistic relationship between the CALP in L1 and L2, as measured by the Woodcock-Munoz Language Survey (WMLS) and reading growth, as measured by Curriculum Based Measurement Oral Reading Probes, with 77 second-and third grade students in transitional bilingual classes. A significant, but weak relationship was found between Spanish CALP Broad Standard Score and English CALP Broad Standard Score with reading growth in Spanish and in English, respectively. The cross linguistic relationship, as measured by the WMLS, and its relationship to reading growth is further discussed. (Contains 4 tables.)