From an enlarged data set of temperature logs and thermal conductivity measurements, surface heat flow (qs) in the Erzgebirge was determined to range from 61 to 112 mW m-2. U-Th-K2O data show that the heat flow pattern is controlled to first order by the occurrence of high heat production Variscan granites within a metamorphic basement. Highest heat flow correlates with granite plutons and sharply decreases off granite. These granites display variable but typically high radiogenic heat production (A), usually between 4 and 10 μW m-3, depending on their chemical type and degree of fractionation. U accounts for 40--90% of the total heat production in the granites, whereas U and Th contribute equally to the radioactivity in the metamorphic basement. Compositional heterogeneity in the upper crust, owing to variation of conductive heat transfer, required corrections of measured surface heat flow up to 20 mW m-2, depending on location of the heat flow site. Heat budget calculations, considering crustal models derived from seismic and gravimetric surveys, define the thickness (D) of crust enriched in radioactive elements to 15 km, the rate of reduced heat flow (q*) to 30--34 mW m-2, and the mantle heat flow to 20--30 mW m-2, in contradiction to what is implied from qs-A plots. In fact, D (5--8 km) and q*(45--52 mW m-2) from these plots represent the thickness of, and the heat flow beneath, the (granite) layer that is most enriched in heat-producing elements. For a crustal section strongly heterogeneous in upper crustal heat production, traditional interpretation of qs-A plots is misleading and should be supplemented routinely by heat budget calculations. ¿ 2000 American Geophysical Union