Isothermal and non-isothermal steady helical flows are theoretically investigated under the assumption that the flow is fully developed in both the thermal and hydrodynamic senses. It is well known that the basic gross characteristics of steady isothermal helical flows of non-Newtonian liquids can be found relatively easily if the flow curve (or non-Newtonian viscosity) in simple shearing is known. Nevertheless, evaluation of more detailed viscoelastic properties in this type of flow is also sometimes desirable. These properties are shown to be exactly determined in both the isothermal and non-isothermal cases as soon as a nonlinear viscoelatic constitutive equation is specified. Shear thinning due to fluid rotation and strong temperature dependence of Newtonian viscosity highly increase dissipative heat. This can produce significant non-isothermal effects in intense helical flows, even when the wall temperature is kept uniform and constant. It is shown that the energy consumption in isothermal and non-isothermal helical flows is always higher than in respective annular flows with the same flow rate. Comparisons between our calculations and available experimental data are also discussed.