The influence of immigrant-background adolescents' heritage language (HL) proficiency and use of the language on parent-adolescent relationships and ethnic identity was investigated in a sample of 414 adolescents from Latin American and Asian backgrounds. HL proficiency, but not language use, was positively associated with the quality of parent-adolescent relationships. Although HL proficiency and language use were both associated with ethnic identity, when taken together, only HL proficiency was a reliable predictor. These findings indicate that it is the development of proficiency in the HL that influences adolescents' successful adjustment, rather than their choice of languages. It is therefore important to support HL development in order to help immigrant-background students to better cope with the stresses of adolescence.