This article describes an experimental study of the void fraction and pressure drop in upflow of air-water mixtures at atmospheric pressure. The experiments were performed with a tube 11 mm in diameter and 2000 mm long over a wide range of phase velocities. The density of the two-phase flow was determined by X-ray techniques. Summaries of engineering equations obtained by other investigators are also presented. The relationship which best approximate the test data herein presented are pointed out.