High-resolution temperature measurements in the main borehole ＇＇KTB-Oberpfalz HB＇＇ of the German Continental Deep Drilling Program are used to determine the undisturbed formation temperature (UFT) by extrapolating the logged values in time using three models: the line source model, the cylindrical source model, and the cylindrical source model with thermal contact resistance. In order to stabilize the inversion algorithm, a priori information about the model parameters must be introduced. The line source model and the simple cylindrical source model yield almost identical results, which, in the lower part of the borehole. lie above the temperature values extrapolated to depth from the almost undisturbed temperature measured in the 4000-m-deep pilot-hole. The simulated UFT obtained from the cylindrical model with thermal contact resistance shows a reduction with respect to the UFT from the other two models in the interval 4000-6000 m for the data from the 6000-m log runs. This difference becomes very small when the measurements from the 7200-m runs are used.