The modified apparent rough surface (MARS) model is presented, which facilitates prediction of liquid holdup and pressure gradient in gas-liquid flow through horizontal and slightly sloping pipes. The model is based on two steady-state, one-dimensional momentum balance equations, which include the superficial velocities and transport properties of gas and liquid, the diameter of the pipe and its angle from the horizontal. Separate correlations are proposed for the interfacial friction factor f(i), the liquid-re-wall friction factor f(L), the interfacial perimeter S-i and the wetted perimeter S-L. Predicted values of liquid holdup and pressure gradient are verified against results of 2400 carefully performed, laboratory experiments in horizontal and sloping glass pipes (-3 degrees less than or equal to beta less than or equal to +6 degrees) of 15, 26 and 51 mm diameter. The liquid holdup epsilon(L) ranges from 0 to 0.42, and the superficial liquid velocity u(LS) from 0 to 0.06 ms(-1) and the superficial gas velocity u(GS) from 1.8 to 34 ms(-1). The gas-liquid systems used are air/water and air/tetradecane (n-C14H30) at atmospheric pressure and room temperature. Significant effects of small inclination angles were found at low gas flow rates, leading to an Eightfold increase in liquid holdup and more than a fourfold increase in pressure gradient compared to horizontal flow. Nevertheless, the average relative error in the prediction of liquid holdup and pressure gradient is less than 10%.