Limited work has been done specifically focussing on the relationship between pressure drop and heat transfer in the transitional flow regime. The purpose of this study was therefore to experimentally investigate and compare the pressure drop and heat transfer characteristics of developing and fully developed flow in smooth horizontal circular tubes in the laminar, transitional, quasi-turbulent and turbulent flow regimes. An experimental set-up was designed, built and validated against data from literature. A smooth circular test section with an inner diameter of 11.5 mm, and maximum length-to-diameter ratio of 872, was used. Pressure drop and heat transfer measurements were taken at Reynolds numbers between 500 and 10,000 at different heat fluxes. Water was used as the test fluid and the Prandtl number ranged between 3 and 7. A total of 317 mass flow rate measurements, 34,553 temperature measurements and 2536 pressure drop measurements were taken. Pressure drop and heat transfer measurements were taken simultaneously and the relationship between pressure drop and heat transfer was investigated. It was found that the Reynolds numbers at which transition started and ended was the same for the pressure drop and heat transfer results. Correlations were developed to determine the relationship between heat transfer and pressure drop, as well as the average Nusselt numbers, in the laminar, transitional, quasi-turbulent and turbulent flow regimes, for both developing and fully developed flow in mixed convection conditions. It was found that the relationship between heat transfer and pressure drop can be used as an additional criterion to distinguish the different flow regimes. (C) 2017 Elsevier Ltd. All rights reserved.