In this paper, we assess the effects of the investigations carried by the Turkish Competition Board in 1997, 2002 and 2003 on the degree competition in the Turkish cement market. For this aim, we used proverbial Bresnahan-Lau framework with alternative definitions for the supply relation. Our first finding is that cement producers had a considerable amount of market power at the period prior to the first investigation in 1997. In addition, this study shows that, parallel to our initial expectation, competition in the cement market increased thanks to the investigations. The positive effect of the first investigation is found to be especially significant. Hence, this study witnesses that the enforcement of the competition law by the Turkish Competition Board has produced the desired effects in the most problematic sector with respect to competition law.