The study considers the prediction of pressure gradients in adiabatic gas-liquid annular two-phase flow in the macro-to-microscale range Twenty-four empirical correlations have been tested against an experimental data bank drawn together in this study containing 3908 points for eight different gas-liquid combinations and 22 different tube diameters, covering microscale and macroscale channels from 0.517 to 31.7 mm in diameter. The correlations of Lombardi, Friedel and Baroczy-Chisholm were found to be the best existing methods when considering macroscale data only, while the microscale database was best predicted by the correlations of Lombardi, Muller-Steinhagen and Heck and the homogeneous model with the two-phase viscosity defined according to Cicchitti. A new correlating approach based on the vapor core Weber number, capable of providing physical insight into the flow. was proposed and worked better than any of the existing methods for the macroscale database This new macroscale method was then extended to cover microscale conditions, resulting in one unified method for predicting annular flows from the macroscale to the microscale covering both laminar and turbulent liquid films. The macroscale method optimized for microchannels worked better than any of the other methods considered (C) 2009 Elsevier Ltd. All rights reserved.