In this study, the influence of a drag-reducing polymer on oil-water flow patterns and pressure gradients was investigated in a downward-inclined pipe (-5 degrees) and compared with the results obtained in a horizontal pipe for the same flow characteristics. Drag reduction was achieved by adding 40 ppm of polymer into an oil-water flow in an acrylic pipe of 0.0306 m ID flowing at 0.1-1.6 m/s mixture velocities and 0.05-0.9 input oil volume fractions. The down-ward inclination enhanced the mixing of the two phases and hence the boundaries between the dispersed and separated flows occurred at lower mixture velocities in downward flow than in horizontal flow. In both pipe orientations, the addition of the DRP affects the flow patterns and pressure gradients mainly in the water-dominated flow regions. Compared with those of the horizontal flow, the effect of polymer addition on flow pattern boundaries was generally lower at -5 degrees inclination. The effect of the polymer on the pressure gradients was more pronounced at horizontal flow than at -5 degrees flow, resulting in a maximum drag reduction of 64% and 55% at horizontal and -5 degrees flows, respectively. Since the polymer used is water-soluble, there were no observable changes in the flow characteristics at the oil-dominated flow regions. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.