The distributed fibre optic temperature sensing technique represents a new physical approach for temperature measurements in geosciences. This method was applied for long-term surveying of temperature variations with time caused by injecting and extracting hot water in a 600-m-deep well. The measuring system was installed as a permanent sensor in the annulus between casing and tubings. The injection/extraction regime was monitored by repeating the measurements every few weeks. Short-term variations of the temperature during injection and extraction were studied by recording the temperature profile every I min over a time period of several hours. Testing shows that the fibre optic temperature sensing technique opens new possibilities for measuring the actual temperature along the entire length of a borehole as well as for monitoring short-term and long-term temperature variations.