Ultra-thin concrete overlays or ultra-thin whitetopping (UTW), are an attractive alternative to traditional practices for the rehabilitation of asphalt pavements. However, it is widely accepted that the exiting pavement should have substantial stiffness to make a UTW a rehabilitation option. This paper re-examines this hypothesis using the performance results and structural evaluation of two experimental sections in Brazil. It is shown that structural contribution of the existing pavement is important for good performance of UTW. However, it appears that the ability of the existing pavement to ensure composite action of the individual slabs in the UTW is more important than its contribution to the flexural stiffness.