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Very limited information is available regarding the impact of heating and cooling processes on the geotechnical performance of piled foundations incorporating pipe loops for ground-source heat-pump systems (so-called energy piles). A pile-loading test that incorporated temperature cycles while under an extended period of maintained loading was undertaken to investigate the behaviour of an energy pile installed in London Clay. Testing was carried out over a period of about seven weeks, with conventional loading tests carried out either side of an extended loading test with thermal cycles. Using an optical fibre sensor system, and other more conventional instrumentation, temperature and strain profiles were observed in the test pile, an adjacent bore-hole, two of the anchor piles, and the heat sink pile. Details of load and movement at the pile head, of ambient air temperature and of the input/output temperature of fluid within the heating system were also recorded. Thermodynamic behaviour observed during the test supports the assumption that the pile acts as an infinitely long heat sink/source, and that the conductivity values used for the London Clay were reasonable. Forces mobilised in the pile shaft and the resistance mobilised at the pile/soil interface have been inferred from the test response, and the effects have been described using a simplified mechanism. Concrete stresses additional to those due to static loading are generated when the pile is heated, and the pile end-restraint conditions influence the effect; concrete stresses could potentially exceed the limiting values imposed by design codes. In this case there was a large margin between the pile ultimate shaft resistance and the shear stresses mobilised at the pile/soil interface during thermal cycling, and as a consequence, it is considered unlikely that the geotechnical capacity of the pile was affected significantly.