New experimental data are obtained for pressure drop and entrainment for annular upflow in a vertical pipe. The 9.5-mm pipe has a hydraulic diameter similar to the subchannels in the fuel assemblies of water-cooled reactors. The test section has a length-to-diameter ratio of 440 to ensure fully developed annular flow. The pressure covers the range from 140 to 660 kPa. Therefore, the density ratio is varied by a factor of ∼4. This allows the investigation of the effect of pressure on the interfacial shear models. Gas superficial velocities between 25 and 126 m/s are tested, extending the range of previous data to higher gas velocities. The data are compared with well-known models for interfacial shear that represent the state of the art. Good results are obtained with the models by Wallis, and Henstock and Hanratty. When the model by Asali, Hanratty, and Andreussi is modified for the effect of pressure, the agreement is also good, and the data collapse with very little scatter. There is a close relationship between these models and mixing length theory such that the models may be viewed as correlations for the surface roughness. This points toward a more fundamental approach in terms of the interfacial structure.