An experimental study is performed for the development of the mean velocity field of swirling turbulent flows in straight pipes with a smooth wall. Swirl intensities were selected in a range that can be expected to occur in large piping systems. Measurements were taken for two different Reynolds numbers, and two widely different initial conditions. Although the swirl intensity can best be expressed by the axial flow of angular momentum, a representation by a single swirl angle seems to be satisfactory for the final decay process. While the difference between the initial conditions persists during a considerable part of the decay process, the decay rates are not essentially different. The rate of decay appears to vary with the Reynolds number in the same way as the friction factor does.