An experimental study was performed to determine the boundary-layer characteristics of an incompressible swirl flow produced by the insertion of a helically twisted tape into a pipe. The resulting flow can be approximated by a uniform axial flow with a superposed forced vortex flow. Boundary-layer velocity measurements indicate that the total velocity in this three-dimensional flow is well approximated by the universal logarithmic velocity profile. Modified axial and tangential logarithmic velocity laws have also been derived and are shown to be in good agreement with the data. The wall shear stress has to be determined either by direct velocity gradient measurements at the wall or by a modified momentum balance since pressure loss measurements do not directly lead to the correct wall shear stress.