Using the nationally representative National Education Longitudinal Study of 1988 (NELS88) database, the relationships of first language (L1) proficiency to subsequent use of that language (home language maintenance), English proficiency and achievement and English self-concept were examined. Confirmatory factor analysis results showed that L1 proficiency in the tenth grade did not correlate significantly with subsequent English grades in school or English proficiency in twelfth grade, but had a strong positive relationship to twelfth-grade use of L1, and a surprisingly positive correlation with twelfth-grade English standardized test scores. Use of L1 up to twelfth grade showed almost no correlation with twelfth-grade English outcomes. Whereas eighth-grade L1 proficiency showed almost no correlation with tenth-grade English outcomes, eighth-grade L1 use had a negative impact on tenth-grade English test and proficiency. However, these negative effects were found only in early high school and did not persist over time. The results did not support speculation that home language proficiency would have persistent negative effects on English outcomes, but suggest the need for support in L1 enhancement for home language maintenance. (Contains 44 references.) (Author/MSE)