To identify possible gender bias in the language of the adults' and children's questionnaires of the Spanish National Health Survey, 2003, its style and language was analyzed for inaccurate generalizations, inequalities due to the use of terms that reflect social stereotypes and hide unequal social roles, and grammatical gender disagreement. Both questionnaires show language bias, mainly lexical, as they use masculine singular nouns to refer to all individuals (for example, carer, employer, worker, doctor, interviewer, interviewee). Gender stereotypes are reinforced by the use of the term "carer", referring to women, and by examples of jobs traditionally done by men. Equally, specific sex denomination for relatives in the same category such as brother/sister is lacking, despite the distinct social roles played by women and men. In conclusion, women are less visible than men in the Spanish National Health Survey, 2003. This study could contribute to the identification and elimination of gender bias in the language of other questionnaires.