The prediction of multiphase pressure drop during the simultaneous flow of oil, gas and water in vertical tubing strings is crucial in the development and optimum exploitation of an oil field. In this paper a method is proposed to improve pressure drop calculations of the Aziz et al. multiphase vertical-flow correlation which has been theoretically justified. The present method suggests combining several flow pattern maps with the Aziz et al. correlation in an attempt to achieve the improvement sought. Two field data sets gathered from 38 production tests conducted in the Middle East and North Africa are used to examine the performance of the various combinations. The results of this work indicate that the performance of Aziz et al. multiphase correlation can be best improved by replacing its original flow-pattern map with the traditional Duns-Ros flow-regime map and for both data sets used. A significant improvement has been observed giving an overall absolute average percent deviation of 2.17% compared with 9.54% for the original correlation. Surprisingly, the Taitel et al. and Barnea et al. flow pattern map does not seem to give accuracies comparable to those of the Duns-Ros correlation. Also it has been found that an improvement in the performance of the Taitel et al. and Barnea et al. flow pattern map would be possible by assigning a value of 0.33 to its void fraction parameter instead of the original value of 0.25. This work represents an addition to the technology of multiphase behavior in vertical pipes and its results will help in a more accurate design of oil well tubing strings in high-rate producing wells.