The temperature structure of the upper part of a subduction zone, i.e. the region between the trench and the volcanic line, has been investigated by means of numerical modelling. Published heat flow measurements are used to constrain the thermal structure of this region and the shear stresses at the plate contact. A pressure and temperature dependent theology is used to model shear stresses. It is shown that temperatures at the plate contact and within the upper plate are primarily determined by frictional heating. Except for the subduction of very young oceanic lithosphere (less than 30 Myr old), these temperatures, as well as shear stresses, are essentially independent of the age of the subducting oceanic lithosphere and the convergence velocity between the two plates. Pressure-temperature conditions at the upper surface of the slab for the proposed thermal model are in good agreement with conditions inferred from high-pressure metamorphic material of regional extent in the Franciscan Complex of California.