Teacher language use has long been debated and the different proportions and functions of the first language (L1) have been recurrently examined. Nevertheless, there has not been enough attention paid to the relationship between teacher language use and learning outcomes using empirical data. Hence, the present study aims to investigate the effect of teacher target language (TL) exclusive use as opposed to L1 use on second language (L2) vocabulary and grammar learning. Data from 60 fourth-year secondary learners from two different groups from a state secondary school in the community of Madrid were gathered by means of pre-tests, post-tests and questionnaires. The two groups were randomly instructed in either the L2 exclusively or through a mixture of the L1 and the L2. In contrast to previous studies, results showed that teaching vocabulary in the L2 exclusively leads to better learning, while it would be advisable to make use of the L2 as well as the L1 when teaching grammar. Based on these results, this paper offers a number of teaching implications to create an effective and successful learning environment.