The advance of new hearing technologies has generated high expectations regarding the development and learning of deaf children, but little research has been done on the language levels of this generation of deaf learners who receive education at the same pace as their hearing peers. The aim of this study was to examine the relationships between academic competence (AC) in Spanish Literacy and Mathematics and linguistic comprehension of deaf Spanish students who are attending Primary Education in general classrooms. Scores on lexical and grammatical comprehension showed that most of them were below their chronological age, and the age gap was greatest for grammatical comprehension. Results showed significant differences both in lexical and grammatical levels when comparing deaf students with different levels of AC. Probing deeper into the relation between grammatical comprehension and AC, certain types of complex grammatical structures have been found related with a better Spanish Literacy. The intervention in linguistic competence should be included in a general programme of intervention in oral and written language.