The purpose of this master thesis is to analyze the possibility of invalidating, due to injury defect, patrimonial legal acts practiced by people with mental or intellectual disabilities. The work seeks to apply invalidity due to injury as an instrument to protect the interests of people with mental or intellectual disabilities, currently considered fully capable, starting from the examination of the changes promoted by the Statute of the Person with Disabilities in the capacity regime, addressing the analysis of vulnerability, the manifestation of will and the impairment of the functionality of such individuals, especially in cases in which the mental disability progressively evolves. The research analyzed the requirements of the injury defect capable of invalidating the patrimonial legal acts and thus protecting the interests of the mentally disabled person who practices the harmful business. Therefore, it is proposed to make the subjective requirement of inexperience more flexible, in order to understand that the person with a mental disability can be considered an inexperienced person and, in this way, compromise their manifestation of will. Finally, a practical application of the reinterpretation of the proposed criterion of inexperience is presented, through the analysis of a hypothetical case of an elderly person who develops mental disability, whose functionality and understanding are compromised for patrimonial legal transactions, thus demonstrating that the injury defect can be applied as an instrument to protect the interests of people with mental disabilities, alongside the support systems provided for in the legal system.