The effect of borehole inclination on ground temperatures and the practical implications it has for the designs of ground-loop heat exchangers (GLHE) systems is studied. We present a general formulation allowing computation of ground temperature for any number of boreholes, each borehole having its own thermal load, dip, direction, depth (of its head) and length. It is shown with an actual design that a slight tilt of the boreholes can substantially improve the theoretical performance of the GLHE. (C) 2009 Elsevier Ltd. All rights reserved.