In this paper, scale up effects have been investigated for three geometrically similar laboratory scale vessels of 0.5, 2 and 201 with retreat curve impellers and cylindrical baffles, which mimic reactors widely used in the pharmaceutical and fine chemical industries, using CFD simulations. The convergence of fully three-dimensional, time-dependent numerical computations has been monitored to ensure the simulations reached quasi-steady state. CFD results have then been validated using LDA measurements and empirical power consumption literature data. The comparisons of power number, discharge flow number, secondary circulation flow number and pumping efficiency at three different scales suggest that the selection of scale of a laboratory vessel has little effect on the macro mixing performance for optimisation of the configuration and operating conditions of an industrial scale reactor. © 2005 Elsevier B.V. All rights reserved.