Leading edge (LE) surface degradation of wind turbine (WT) blades caused by insect accumulation, erosion and other environmental agents reduces the aerodynamic performance of the blades, causing WT power and energy yield losses. Estimating these losses is paramount for cost-informed maintenance planning. Computational Fluid Dynamics (CFD) can predict aerodynamic performance losses. However, sensitivity of these predictions to physical model choice and detailed model settings can be large. To assess this sensitivity, the International Energy Agency Task 46 – Erosion of Wind Turbine Blades, developed the First Aerodynamic Benchmark, presented herein. The performance degradation of the NACA 63<sub>3</sub>-418 airfoil due to moderate and severe LE degradation, assessed experimentally in two wind tunnel measurement campaigns, is studied. The clean and degraded airfoil performance predicted by seven CFD codes and two low-fidelity methods are cross-compared and benchmarked against measurements. A utility-scale WT featuring the NACA 63<sub>3</sub>-418 airfoil on the outboard blade is used to determine the resulting power and energy losses onshore and offshore. Most codes succeed in predicting the measured airfoil performance reduction due to moderate LE degradation before stall. Consequently, all energy loss estimates are close. Conversely, the variability of the predicted aerodynamic performance reduction due to severe LE degradation is larger, and the variability of the resulting energy losses is also larger than at moderate LE degradation. These results underline both the significant sensitivity to the specific analysis set-up and the need for further research into methods for predicting the impact of advanced LE degradation, such as geometry perturbation-resolving simulations.