The influence of immigrant-background adolescents’ heritage language proficiency and use of the language on parent-adolescent relationships and ethnic identity was investigated in a sample of 414 ninth-grade participants from Latin American and Asian backgrounds. Heritage language proficiency, but not language use, was found to be positively associated with the quality of the parent-adolescent relationship, especially for Asian American adolescents. Although heritage language proficiency and language use were both associated with strength of ethnic identity, when taken together, only heritage language proficiency emerges as a reliable predictor of ethnic identity. These findings indicate that it is the development of proficiency in the heritage language that influences adolescents’ successful adjustment, rather than their own choice of languages.