Data are presented on the total two-phase pressure drop for air-water vertical flow in a 0.026 m i pipe. The data exhibit a maximum value, at the end of the churn flow regime, set between two minima at the end of the slug and annular ripple regimes respectively. There was a noticeable diameter effect. It was possible to explain the form of the pressure loss data qualitatively. In the slug and allied regimes the rising gas bubble carried a pocket of Liquid in its wake resulting in a reduction in the fluid head.None of the current theories of pressure loss prediction proved to be useful. However, it was possible to predict bubble flow at low gas rates and annular droplet flow at high gas rates.