This study aims to verify the effect of implementing a lean manufacturing (LM) system on operational performance in an industrial company's production line. There are few studies that seek to quantitatively verify the effect of implementing an LM system on the operational performance of a company, and that is the main objective of this research. In this casebased study, the LM implementation in the company studied is compared with best practices and reference models, ascertaining the degree of adherence of the company's program. The operational performance, in terms of three key dimensions of cost, quality and lead time, is compared before and after the implementation of the LM program. By comparing the means of quantitative indicators related to the three performance dimensions, evidence was found of significant gains in the operational performance, and it was demonstrated that the company followed many of the best practices of LM implementation found in the literature. This study contributes to managers and to the literature on LM by measuring its apparent impacts on operational performance.