The primary objective of this paper is to illustrate the inherent problems of measuring the apparent viscosity of cement slurries with laboratory viscometers and using these measurements to estimate friction factors and flow regimes in large‐scale pipes. Experimental measurements obtained with Couette and tube viscometers are shown to exhibit an apparent wall slip that reaches a limiting value of 100% at shear stresses near the yield value. Nevertheless, comparable apparent viscosities are obtained after these data are corrected for slip. Experimental results for the laminar, transitional, and turbulent flows of clay and cement slurries in a large‐diameter pipe are also presented. It is shown that the onset of turbulence occurs at a critical Reynolds number that is in good agreement with the predicted value, provided that rheological properties are based on large‐scale pipe‐flow data. However, for yet unexplained reasons, samples of cement taken from the large‐scale facility had much higher apparent viscosities (as determined from a Couette viscometer) than viscosities calculated from laminar pipe‐flow data.