The actual value of the heat rejection through the cooling towers from the air-conditioning system of the typical large-scale office building is measured in the tropical summer climate of Shenzhen. The daily average measured value of the total heat rejection per unit building area is 82.75 W/m(2) during weekdays. During the weekend, this value is only 20.24W/m(2), i.e., about 1/4 of that found for the weekdays, which is attributable to the less activity of building. The proportion of the sensible heat rejection is only 1.15%, and even attains a negative value when the inlet water temperature is lower than the inlet air temperature of the cooling towers. To evaluate quantitatively the effect of the heat rejection from the air-conditioning system on the urban thermal climate, the heat-rejection efficiency ratio (HRER) is presented in this study, for which the average measured value during weekdays is 1.35. In addition, a heat-rejection numerical model is developed, and the heat-rejection simulation results generally agree well with the field test data. The results indicate that the hot and humid climatic conditions impair the heat and the mass transfer efficiency of the cooling towers, and ultimately reduce the heat rejection from the building air-conditioning system. (C) 2015 Elsevier B.V. All rights reserved.