Hollow fiber membranes are used widely in membrane separation processes because of their high surface area to volume ratio. Several models have been developed for the hollow fiber spinning process. However, many details of the process are not well understood. In this work, the conservation of mass and momentum equations are solved for isothermal, axisymmetric hollow fiber spinning. In contrast to past work, the conservation equations are solved for both the bore and clad fluids. The two sets of equations are linked by continuity of velocity and stress across the bore-clad interface. Simulation results show an unexpected recirculation region in the bore fluid under certain operating conditions. For a given die geometry, the presence/absence and size of the recirculation region is dependent on the bore-to-clad flow rate ratio and bore-to-clad viscosity ratio. The appearance of the recirculation region leads to a more rapid decrease of both fiber outer and inner radii after die swell. The predicted changes in fiber radii are in better agreement with experimental observations than predictions from the one-dimensional thin filament analysis. (c) 2006 American Institute of Chemical Engineers.