This article reports the results of an experimental study of frictional pressure drop for two-phase flow (air and water) inside horizontal spirally fluted tubes. Nine spirally fluted tubes, representing a wide range of geometric parameters, were tested adiabatically with various flow rates of air and water. Average pressure drop enhancement levels (our predicted smooth tube values) in the range of 1.4 to 3.9 were observed. A design correlation was developed based on modifications to an existing smooth-tube model. The proposed correlation predicts 84% of the database (581 experiment data points) within +/- 20%.