Measurements of film cooling on a simulated turbine blade are conducted using a mass transfer technique. Under the influence of the endwall, dramatic changes of film cooling performance occur on the convex surface of the blade as compared to the region where the flow is two dimensional. The result is a triangular region, where coolant is swept away from the surface by the three-dimensional vortex-driven flow present between adjacent blades. In order to predict the area of this unprotected region, the influences of several parameters including density ratio, blowing rate, and number of rows of injection holes are studied. The presence of the endwall affects the film cooling performance on the concave surface only slightly.