We calculate the influence of thermal mass and night ventilation on the reduction of the maximum indoor temperature in summer in residential buildings without using air-conditioning. The results are given for different locations in the hot-humid Mediterranean climate of Israel. We found that the maximum obtained indoor temperature reduction depends linearly on the temperature difference between day and night at the site. The results obtained show that one can predict the indoor temperature decrease, due to the thermal mass and night ventilation, from the simple and available data of the long term average temperature swing of the site, which is a common available data. The conclusion is that in the hot-humid climate of the Mediterranean coast, high thermal mass with night ventilation is effective for residential buildings with no air-conditioning provided the temperature swing is above 5degreesC, which is in general the case in this climate.