The aim of this paper is to discuss the impact of the relation between varying indoor and outdoor conditions on the ventilation loads of buildings and to provide HVAC designers with the respective information needed for the optimum dimensioning of the system. The total load generated by one litre per second of fresh air brought from the outside environment to the indoor space conditions, called -ventilation load index-, is calculated for the cities of Athens and Thessaloniki, Greece. The same principles can be applied to other locations. Results are given in tabular form and ventilation loads for both heating and cooling periods are presented. The results separate the sensible and latent components of the load in order to provide the latent to sensible ratio. Operation periods of 8, 16 and 24 hours are considered for the air-conditioning system, in order to distinguish between different types of buildings. Various combinations of indoor conditions are also examined, in order to ascertain the influence of the temperature and relative humidity on ventilation loads. The resulting energy requirements due to the ventilation of the building are synoptically presented in tables.