An investigation has been conducted of the three-dimensional flow associated with the inlet-vortex phenomenon. Quantitative measurements were made of inlet– and trailing-vortex circulation and position. Flow visualization was used to obtain information on the overall structure of the vorticity field. It is shown that the inlet vortex and the trailing vortex have essentially equal and opposite circulation. In addition, the production of vorticity, due to vortex stretching, is the mechanism by which both of these are maintained. A limited parametric study was also conducted to define the circulation and position (in the inlet) of the inlet vortex, for parameters of practical engineering interest.