A theoretical model is developed to determine the flow characteristics for vertical liquid jets of annular cross section. The fully implicit numerical marching integration procedure is based on the parabolic flow theory of Patankar and Spalding. The method is more accurate and general than previous analytical models, since it accounts for all significant forces acting upon the jet and for complex boundary conditions, such as an ambient pressure difference across the jet or a non-uniform velocity profile at the inlet. Comparison of the results with existing experimental data indicates reasonable agreement, and the predictions correspond closely to those of the earlier theories for simplified flow cases.