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. However, the extent to which immersion education fosters bilingualism 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 third and eighth grades. The authors found that by eighth grade, the average dual-language immersion student, regardless of language, performed at least at the Intermediate Low sublevel, and often higher, on Standards-Based Measurement of Proficiency assessments of nearly all language skills tested (listening, reading, writing, and speaking). In comparison, eighth 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 sublevel. After four years of immersion learning (grades K­-3), fourth-grade students whose home language was Spanish scored similarly in reading and speaking to their immersion peers whose home language was not Spanish; however, they outperformed their immersion peers in listening and writing.