The aerodynamic influence of casing surface roughness on over-tip-leakage flow was investigated in a large scale, rotating, axial turbine rig. Phase-locked measurements of the absolute total pressure in a cold flow turbine research facility were conducted at the turbine stage exit using a high-frequency-response total pressure probe. Time accurate measurements provided valuable aerodynamic information quantifying the near tip flow modifications imposed by artificially roughened casing inner surface. A partial segment of the turbine casing was roughened by using a roughness layer of two different mean roughness heights. A smooth wall as a baseline case was also investigated by attaching a smooth layer of equivalent thickness to the casing surface. Artificially roughening the casing surface significantly reduced the leakage mass flow rate and the momentum deficit in the core of the tip vortex. The reductions obtained in the tip vortex size and strength influenced the tip-side passage vortex and other typical core flow characteristics in the passage. The influence of casing roughness was studied in a range of tip clearance values.