This study was conducted to explore the effects of a two-way immersion bilingual program on maintenance/acquisition of Spanish-language proficiency and on reading and mathematics achievement in English over a period of 4 academic years. The researchers used Hierarchical Linear Modeling (HLM) techniques to compare the effects of two different program models on Spanish-language proficiency of participating students from Spanish- and non-Spanish-language backgrounds. In addition, they employed multivariate matching algorithms to construct a comparison group of schools and students that matched program students academically and demographically and then used HLM methods to compare academic achievement in reading and mathematics of students in program and comparison groups. The researchers found that students in the program model that offered Spanish instruction in language arts and one content area performed better in reading comprehension in Spanish than students in the program model that offered only Spanish language arts instruction. The researchers also found that program students exhibited achievement levels in reading and mathematics that were on par with or higher than those of demographically and academically similar students not in the program. In addition, the researchers determined and that average annual learning rates in both academic disciplines were similar for program and comparison students. (Contains 5 figures, 2 footnotes, and 3 tables.)