The moderate Reynolds number flow developing in a cylindrical unbaffled stirred tank is studied using direct numerical simulation (DNS) and the results are compared with available experimental data and Reynolds Averaged Navier-Stokes (RANS) solutions. The geometry of the impeller is handled using an immersed-boundary procedure implemented in a solver written for cylindrical coordinates. This allows efficient simulation of the flow at a reasonable computational cost and accurate prediction of the mean and rms velocity fields. For this configuration, RANS performs poorly because of the low Reynolds and strongly unsteady and inhomogeneous nature of the flow: many different flow structures are produced, ranging from small-scale vortices generated at the blade tips to large-scale meridional recirculation. It is shown, in addition, that inaccurate results are produced by the wrong (computational) assumption that an impeller with n blades produces instantaneous fields with an n-fold symmetry. Because simple stirred-tank configurations (like the present one) have been recently used to assess the performance of several RANS closures, the main message of this study is that simple geometrical configurations and low Reynolds numbers are not benign parameters for such a task. (C) 2004 American Institute of Chemical Engineers.