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This paper intend to answer the following research question: what is the constitutional principle upheld by the Supreme Court in decisions on social security matters inside Brazilian abstract constitutionality control? The right to social security is has a status of fundamental right in the Brazilian Constitution, but it also is a costly provision for the State. The Legislative and Executive branches periodically change the pension rules in order to contain costs. As a contramajoritarian power, the Judiciary has a duty to protect the citizens’ social rights by preventing that decisions taken by the other branches fall back (or terminate) rights guaranteed by the 1988’s constitution. The objective of this study is to understand whether the Supreme Court fulfills that role in social security matters. Therefore, we carried out a descriptive exploratory analysis with small “n” on the Unconstitutionality Direct Actions’ decisions filed against the Constitutional Amendment numbers 20/1998 and 41/2003. The result we found was that the Supreme Court, in the analysed decisions, was more concerned to ensure financial and actuarial balance of the system than protect the rights envisaged originally in the 1988’s constitution.