Purpose - The purpose of this paper is to illustrate an application of the Six Sigma define, measure, analyze, improve, control (DMAIC) problem-solving methodology in a Portuguese industrial enterprise, aimed at reducing the rejection rate of a specific manufactured product due to defects generated in an aluminum gravity die casting operation. Design/methodology/approach - Along the five-stage roadmap, a set of analytical and conceptual tools was systematically employed to better characterize the problem, define the product's critical to quality characteristics, estimate the process baseline, determine the relevant cause-and-effect relationships, identify the root causes leading to the high rejection rate, implement an improvement plan, determine the effectiveness of the improvement actions, and put in place a control plan. Findings - The project team discovered that the high rejection rate was caused by factors inherent to the gravity die casting operation itself and by the mixing of rejected parts made of different types of alloys from other locations within the plant. Another key finding was that the successful execution of the project was only possible due to the belief and support of top management and to the active involvement of the team members. Practical implications - This case study illustrates a successful practical application of a Six Sigma project in a small-medium enterprise in Portugal, as well as the operational and financial benefits that derived from it; thus providing a good example for others to follow. Originality/value - Few cases of continual improvement initiatives, including Six Sigma projects, developed at a Portuguese company are available in the literature; this paper fills such void by describing a very successful application that demonstrates the advantages that other companies can learn in terms of adopting structured methodologies to improve the quality of their products and the efficiency of their processes. Moreover, the analysis and conclusions herein presented can be of great importance for companies using gravity die casting technology.