This paper describes measurements of the change in surface roughness of aluminium strip due to cold rolling. Rolling is in the mixed lubrication regime, where there is both asperity contact and hydrodynamic action. The strip is in the as-received condition before rolling, with a continuous spectrum of roughness wavelengths. The spectra of roughness for both the initial and rolled surfaces are used to extract amplitudes for long and short wavelength components, with an arbitrary division at a wavelength of 14 μm between these components. It is found that the short wavelength components persist more than the long wavelength components, and that flattening of the strip increases with reduction in strip thickness. The qualitative effect of wavelength on flattening is similar to that observed with unlubricated rolling (Sutcliffe, 1999), and is in line with theoretical models of mixed lubrication. The effect of reduction is not predicted by existing theories, but is in agreement with measured variations of friction with reduction.Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in Orlando, Florida, October 11–13, 1999