The liquid hold-up epsilon (L) and pressure gradient (-dP/dx)(Tp) occurring during steady-state (nearly) horizontal co-current gas-liquid pipe flow have been calculated using the general momentum balances for both phases, two different models for the wall shear stresses and 22 different correlations for the interfacial friction factor. The calculated results are compared with an experimental database of the University of Amsterdam, consisting of 3981 measurements of gas-liquid pipe flow in three flow regimes, i.e. stratified, wavy and annular flow, and the following process conditions: pipe diameters 0.0127 < D-i/m < 0.0953, lengths 11 < L/m < 22, angles of inclination -5 < (T)/degrees < 6 and viscosities 8.52 x 10(-4) < etaL/(Pa.s) < 0.092, densities 996 < rho (L)/(kg m(-3)) < 1220, surface tensions 0.038 < sigma/(Pa.m) < 0.073. The database comprises data of Andristos. Furthermore, newly acquired experimental data is presented, obtained with a newly constructed experimental gas-liquid flowloop. This data adds to the public domain and may be used by other researchers to develop and test newf(i)-relations. The authors found that a newly developed f(i) model based on the interfacial wave velocity C, presented in this paper, gives the best simultaneous prediction of both the liquid hold-up and the pressure gradient. The f(i) model of Andritsos and Hanratty(1) combined with the model for the wall friction factors f(G), f(L) of Taitel and Dukler(2) calculates the most accurate value of the pressure gradient. The f(i) model of Grolman and Fortuin(3) combined with the wall ftiction factors f(G), f(L) of Hart et al.(4) calculates the most accurate value of the liquid hold-up.