Thermal conductivity is a key parameter in the design of borehole heat exchangers. Thermal response tests are becoming increasingly more popular for measuring in situ thermal conductivity but no theoretical investigations have been done so far that account for three-dimensional effects. We compared the results from a 3-D finite-element numerical model with those of a simple analytical line-source solution and tested their sensitivity to the duration of the tests. The effects of heterogeneous subsurface conditions, groundwater movement, and variable data quality are presented. Comparison with measured data emphasizes the importance of using more sophisticated numerical methodologies in interpreting thermal response test data. (C) 2006 CNR. Published by Elsevier Ltd. All rights reserved.