The introduction of any regulatory model must be able to balance the trade off that exists between consumer and investors interest, aiming an adequate performance for the sector, increasing efficiency and generating volumes of investments enough to sustain long term growth in the economy. The regulatory reform in Brazilian Telecommunication sector can be considered a case of success internationally, which can be seen due to more than 100 billion of reais invested since then, the availability of products and services, quality improvements, and due to the relevance Brazil assumed in the international telecommunication scenario. However, since the beginning of the current Government, constant disagreements between the Telecommunication Ministry and ANATEL brought an instable environment to the sector. Since February 2003 it started strong discussions regarding contracts, criticism to the way the regulatory agency used to act as a whole and, particularly, the fixed telecoms tariffs readjustments. At that time, the Government considered the substitution of IGP-DI as the index for fixed telecoms readjustments. The objective of this study is, in light of the dispute between Government, ANATEL and fixed telecom companies in 2003, to investigate if this episode brought any impacts to the return of the most liquid shares of fixed telecom companies at the São Paulo Stock Exchange. The Event Study Methodology was chosen to proceed with this study, which is widely used in Finance due to its strong general applicability. Additionally, it was used the methodology proposed by Izan (1978), aiming to address the additional difficulty brought by the fact that this is a regulatory event.