In this study, the effect that the top boundary has on the dow in the bottom region of a fully baffled stirred vessel of standard geometry, was investigated. Measurements of the three dimensional mean and fluctuating velocity components were made with Laser Doppler Anemometry. Calculations of the flow field were made with the commercially available CFD code FLOW3D. Three cases were studied. Two in which the top boundary was located at 30 mm, one with an air-liquid interface (open vessel) and one without (closed vessel). For the third case a closed vessel was also used and the top boundary was located at 150 mm.The effect of removing the air-liquid interface at 300 mm on the how in the bottom region of the vessel was small enough to be within the experimental uncertainty. Lowering the closed top boundary from 300 to 150 mm had a rather small impact on the flow field in the lower part of the vessel. It was concluded that the detected change is due partly to a direct effect from the introduced lid and partly to the fact that the impeller discharge is affected. The observed changes are so small that it can safely be stated that the suspension capacity of the system is not affected when the top boundary is altered within the limits of the present investigation.