The present study examined whether full access to sign language as a medium for instruction could influence performance in Theory of Mind (ToM) tasks. Three groups of Italian participants (age range: 6-14 years) participated in the study: Two groups of deaf signing children and one group of hearing-speaking children. The two groups of deaf children differed only in their school environment: One group attended a school with a teaching assistant (TA; Sign Language is offered only by the TA to a single deaf child), and the other group attended a bilingual program (Italian Sign Language and Italian). Linguistic abilities and understanding of false belief were assessed using similar materials and procedures in spoken Italian with hearing children and in Italian Sign Language with deaf children. Deaf children attending the bilingual school performed significantly better than deaf children attending school with the TA in tasks assessing lexical comprehension and ToM, whereas the performance of hearing children was in between that of the two deaf groups. As for lexical production, deaf children attending the bilingual school performed significantly better than the two other groups. No significant differences were found between early and late signers or between children with deaf and hearing parents.