Polypropylene hollow fibers were prepared via melt spinning at speeds of 1000-2500 m/min. The outside diameters of the fibers were measured on-line with high-speed photography. The fiber formation process was modeled with momentum, energy, and two continuity equations (one for the polymer, and one for the lumen fluid). The equations were solved numerically, and the results were compared to the on-line diameter data. Both Newtonian and viscoelastic constitutive equations were considered.