This paper presents an improved model for gas-liquid two-phase flow in the churn and annular flow regimes for small- and large-diameters in vertical and near-vertical pipes. Many investigators consider churn flow to be the least understood flow regime in upward gas-liquid flows in vertical pipes. Nevertheless, this flow regime occurs commonly in several applications in the oil and gas industry, such as: gas-lift operations, production of gas-condensate wells, and two-phase flows with high gas-liquid ratio in general. Additionally, the accuracy of two-phase flow models for large pipe diameters, irrespective of flow regimes, is still questionable. Only a limited number of studies can be found in the open literature that validates two-phase flow models for pipe diameters larger than 0.203 m (8 in.). The model developed in this study proposes a modification to an approach originally proposed in previous studies for pipe diameters smaller than 0.0508 m (2 in.). These modifications have proven to improve accuracy in the prediction of pressure gradient and liquid holdup for small and large pipe diameters under churn and annular flow conditions. The absolute average error in the prediction of experimental pressure gradient decreases from 155% (for the original model) to 36% (for the modified version). This study validates the proposed model with field and laboratory experimental data from several different studies from the literature, in terms of pressure-gradient and liquid holdup results, for pipe diameters ranging from 0.0318 to 0.279 m (1.25-11 in.), pressures from 1 to 613 bara (14.6-8900 psia), and fluids types such as air-water and oil-natural gas systems. This study also compares its results with other widely accepted models in the commercial packages. When compared to field and laboratory data, the simulation results show that this new model has an overall better performance compared to other models widely used in the oil and gas industry. (C) 2016 Elsevier Ltd. All rights reserved.