Hybrid ventilation is one promising approach to reduce energy consumption in office buildings. On the one hand, a minimum air change rate is supplied to the rooms, even if the windows are closed. On the other hand, the energy demand for ventilation can be reduced if natural forces (wind and buoyancy driven air flow) are used to ventilate the building. The user behaviour has an important but often unknown influence on the thermal building performance and the indoor climate. Thus, an accurate user model should be used in designing hybrid ventilation. This paper focuses on the data evaluation of measurements at the institute building of Fraunhofer ISE [SolarBau:Monitor (2004 a)] concerning user attendance and user behaviour with regard to the use of blinds, equipment and windows in 16 offices. The stochastic user behaviour is derived from a data analysis, is taken into account by a Monte Carlo-simulation and is compared with the assumptions from the design phase.