This paper presents a two-step analysis of night ventilation as a way of cooling office buildings and providing comfort in summer. Experimental data first allows us to discuss some factors which affect the performance of the technique, to show that significant comfort improvement may be obtained in ＇＇well-designed＇＇ rooms, and to investigate the energy removal from the building by defining a potential energy efficiency index. Through the use of numerical simulations, we then deal with the useful cooling energy which is offered by night ventilation in the experimental configuration and show that much care has to be taken when the technique is intended to be used in the frame of a mixed-mode cooling system.