The observed linear relationship between surface heat flow (q0) and the radiogenic heat production of near surface rocks (A0) discovered almost two decases ago has been a primary tool used to infer the distribution of heat production in the continental crust. Recent evidence on the nature of compositional heterogeneities in the continental crust has led us to reexamine the q0-A0 relationship and the likely implications for the thermal structure of the lower continental crust. Numerical simulations of crustal thermal models indicate that some interpretations deived from this relation may be flawed. In particular, the length scale determined from the q0-A0 function, normally thought to describe the depth disgtribution of crustal heat souces, is significantly affected by the horizontal scale of crustal heterogeneity. As a consequence, most estimaes of lower crustal heat flow and lower crustal temperatures obtained using heat flow data with the q0-A0 relationship are overestimates. This overestimation is (proportionally) largest in areas of low mantle heat flux such as cratons, where basal heat flux may be overestimated by more than a factor of two. ¿ American Geophysical Union 1987