This study investigated the pressure drop characteristics in microtubes using R-134a as a test fluid. The test tubes were the circular stainless steel tubes with inner diameters of 0.244, 0.430, and 0.792 mm. Although some of the existing studies reported the early flow transition at the Reynolds number of less than 1000, it was not found in the single-phase flow pressure drop tests. The conventional theory predicted the friction factors well within an absolute average deviation of 8.9%. The two-phase flow pressure drop increased with increasing quality, increasing mass flux, and decreasing tube diameter. The existing correlations failed to predict the two-phase friction multipliers in the microtubes of this study. A new correlation to predict the two-phase flow pressure drop in microtubes was developed in the form of the Lockhart-Martinelli correlation. It includes the effect of the tube diameter, surface tension effect, and the effect of the Reynolds number on the two-phase flow pressure drop in microtubes. The new correlation developed in this study predicted the experimental data within an absolute average deviation of 8.1%. (c) 2005 Elsevier Ltd. All rights reserved.