ESDU 04006 is one of a group of Data Items on the prediction of pressure drop in two-phase gas-liquid mixtures in piping systems. It provides techniques for the assessment of the frictional and gravitational pressure gradient for fluids flowing upwards in smooth vertical pipes. This assessment requires a knowledge of the phase flow rates@ phase physical properties and pipe diameter. It also provides methods for estimation of the accuracy of the predictions. Within ESDU 04006@ the basic generic methodologies for the empirical prediction of total pressure gradient are reviewed. A description is then given of the development of a database of data on vertical two-phase flows. This database contains nearly 6000 carefully selected and evaluated data points and is comparable in size with previous databases. In the validation process@ many more thousands of data points were rejected as being unreliable. The performance of a number of correlations for void fraction (hold-up) and frictional pressure gradient was tested against the database@ and performance comparisons underpins a methodology for the selection of the best-performing correlation under given flow conditions. The results of the performance comparisons with the database are reviewed and the methodology for applying the results is described. Worked examples illustrate the selection methodology and the calculation of total pressure gradient for air-water and steam-water flows.