Medium speed diesel engines are well established today as a power source for heavy transport and stationary applications and it appears that they will remain so in the future. However, emission legislation becomes stricter, reducing the emission limits of various pollutants to extremely low values. Currently, many techniques that are well established for automotive diesel engines (common rail, after treatment, exhaust gas recirculation - EGR, …) are being tested on these large engines. Application of these techniques is far from straightforward given the different requirements and boundary conditions (fuel quality, durability, …).This paper reports on the development and experimental results of cooled, high pressure loop EGR operation on a 1326kW four stroke turbocharged medium speed diesel engine, with the primary goal of reducing the emission of oxides of nitrogen (NOx). Measurements were performed at various loads and for several EGR rates. The effects of EGR on the combustion are discussed by analyzing the apparent heat release rates. Afterwards this is related to the effect of EGR on the emissions and performance. The results reveal that EGR causes a shift of the combustion towards a later phase, which results in an important reduction of NOx emissions but also a significant increase of CO emission and fuel consumption.