A suite of 20 heat flow measurements has been completed across the Intermontane and Omineca Crystalline belts in south-central British Columbia at about 50°N. Values along the 200 km line are high (83 mW m−2, corrected for Pleistocene glacial climate; reduced heat flow is 67 mW m−2) and uniform (standard deviation = ± 10%). There appears to be no difference in the thermal structure of the two geologic belts. Two sources of heat are considered to explain the level of heat flow observed: a discrete thermal event in the Eocene, and a steady-state supply of heat maintained in the back-arc location by asthenospheric flow caused by nearby subduction. Both can account equally well for the elevated heat flow observed. However, in light of seismic, magnetic, electrical, and flexural data that suggest that the lithosphere may be as thin as 30–40 km, it is concluded that a steady supply of heat must exist since this thickness is much less than the thickness of lithosphere that would be present 50 Ma after even a major thermal event.