Measurements of the drop size distributions were obtained for air and water flowing in an annular pattern in a 2.54 cm horizontal pipe. A laser diffraction technique was used. The data were obtained at the centerline. These are compared with measurements obtained previously for annular flow of air and water in a horizontal 9.53 cm pipe in the same flow loop and with the same instrumentation. The volume median drop diameter for these two flows was found to increase with pipe diameter and to vary with D-0.5 for superficial gas velocities larger than 30 m/s. (C) 2002 Elsevier Science Ltd. All rights reserved.