Research has demonstrated that students in dual-language immersion programs perform as well as, or better than, their peers in core academic content areas by late elementary school. The extent to which immersion education fosters bilingualism, however, has received less attention in the literature. Using data from a four-year efficacy study of dual-language immersion education in the Portland Public Schools in Oregon, this study reports the skill levels that 1,284 dual-language immersion students achieved in their classroom partner languages (Spanish, Japanese, and Mandarin Chinese) between 3rd and 8th grades. The authors find that by 8th grade, the average dual-language immersion student, regardless of language, performs at least at the Intermediate Low sub-level, and often higher, on STAMP assessments of nearly all language skills tested (listening, reading, writing, and speaking). In comparison, 8th graders in the Portland Public Schools who began taking Spanish as an elective in upper elementary or middle school scored only at about the Novice Mid sub-level. After four years of immersion learning (Grades K-3), 4th grade students whose home language was Spanish scored similarly in reading and speaking to their immersion peers whose home language was not Spanish; they outperformed their immersion peers, however, in listening and writing. [This paper was published in "Foreign Language Annals" (EJ1115027).]