The present contribution reports on a direct numerical simulation (DNS) of a turbulent boundary layer that undergoes separation due to the presence of a pressure gradient at a moderate Reynolds number. The variation of the free-stream velocity has been chosen according to an experiment of Kalter and Fernholz (The influence of free-stream turbulence on an axisymmetric turbulent boundary layer in, and relaxing from, an adverse pressure gradient. In: 5th European Turbulence Conference, Siena, 1994). Because of limited computational resources, the momentum thickness Reynolds number at the reference position had to be reduced to Reθ=870 which is about half as large as that of the experiment. Nevertheless, a comparison of integral parameters as well as first and higher order moments shows fair agreement between DNS and experiment. An inspection of the momentum balance underlines the prominent role of the Reynolds stresses in controlling the structure and geometry of the separation bubble. Its shape and dynamics are governed by large scale vortices reaching from the wall to the shear layer above it.