Three-dimensional helically dimpled tubes have been experimentally studied in order to obtain their heat transfer and isothermal friction characteristics. Using water and ethylene glycol as test fluids, a wide range of fluid flow conditions was covered: 2000 < Re < 100,000 and 2.5 < Pr < 100. An experimental study of 10 tubes with different geometric forms (dimple height h/d ranging from 0.08 to 0.12 and helical pitch p/d, from 0.65 to 1.1) offers insight into the influence of manufacturing parameters on tube thermohydraulic behaviour. The large amount of experimental data have been correlated so as to obtain easy to use expressions for Fanning friction factors and Nusselt numbers as functions of flow and geometry non-dimensional parameters. Performance evaluation criteria, commonly used in the enhanced heat transfer literature, were calculated in order to assess the real benefits offered by dimpled tubes.