Homogenous oil in water dispersion has been investigated in a horizontal pipe. The mean droplet size is 25 mu m. Experiments were carried out in a 7.5-m-long transparent pipe of 50-mm internal diameter. The wall friction has been measured and modeled for a wide range of flow parameters, mixture velocities ranging from 0.28 to 1.2 m/s, and dispersed phase volume fractions up to 0.6, including turbulent, intermediate, and laminar regimes. Flow regimes have been identified from velocity profiles measured by particle image velocimetry in a matched refractive index medium. It is shown that the concept of effective viscosity is relevant to scale the friction at the wall of the dispersed flow. Based on mixture properties, the friction factor follows the Hagen-Poiseuille and the Blasius' law in laminar and turbulent regimes, respectively. Interestingly, the transition toward turbulence is delayed as the dispersed phase fraction is increased. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 1119-1131, 2011