This study summarizes the cooling performance of a ground-source heat pump system which was installed in a 65-m(2) room in the Solar Energy Institute, Ege University, Izmir (568 degree-days cooling, base: 22degreesC; 1226 degree-days heating, base: 18degreesC) Turkey. The institute, built in 1986 has a liveable floor area of 3000 m(2), and uses passive solar techniques. The heating and cooling loads of the room were, respectively, 3.8 and 4.2 kW at design conditions. The system was commissioned in May 2000 and the performance tests have been carried out since then. Based upon the measurements the heat rejection rate to the soil with an average thermal diffusivity of 0.00375 m(2) h(-1) in the cooling mode was found to be in average 51 W m(-1) of bore depth, while the maximum entering water temperature to the unit was recorded as 35.9degreesC. The cooling coefficient of performance of the heat pump and the whole system was relatively low when compared to other heat pumps operating under conditions at or near design values. The primary reasons for this were discussed in detail and the potential for performance improvements was also suggested. Copyright (C) 2002 John Wiley Sons, Ltd.