The purpose of the study was to measure the performance of natural ventilation in 5- and 9- storey apartment buildings, and to offer solutions for the improvement of ventilation. In the four most common building types, the functioning of the old ventilation system was measured during a spring and summer period. The economically feasible improvement solutions, as installing the auxiliary fans and mechanical exhaust ventilation, were analysed with measurements and calculations. The old natural ventilation system with stacks and leaky windows can maintain a certain basic level of ventilation.