Gas-liquid downward flow is frequently encountered in hilly-terrain pipelines, down-comer pipes extending from offshore production platforms to sea floors, and steam injection wells in thermal enhanced recovery operations. Since the gravitational and frictional terms in the total pressure gradient equation Sor downward flow have opposite signs, gas-liquid downward flow in inclined and vertical pipes may experience either pressure loss or pressure gain, depending on flow rates, pipe geometry, and fluid properties. A mechanistic model for the pressure loss/gain boundary of gas-liquid downward flow in inclined and vertical pipes is developed iii this work and is verified with available experimental results. The effects of pipe inclination angle, inside diameter wall roughness, and fluid physical properties on the pressure loss/gain boundary are presented. [S0195-0738(00)00302-2].