The present study shows how the application of computational fluid dynamics can help to understand and optimize the flow field in a passenger compartment in order to achieve an optimum of thermal comfort for the occupants.The flow field and temperature distribution in a passenger compartment have been calculated using the commercial CFD program STAR-CD. In combination with a thermophysiological model for the passengers, the computational results are used to evaluate the thermal comfort of the occupants and compare different geometrical modifications. The computational mesh consisting of around 3 millions hexahedra cells resolves all geometrical details of the car cabin including the air ducts, air nozzles and louvers. Natural convection, heat conduction and radiation are taken into account. One standard climatisation mode, the winter heat-up mode has been simulated. A special emphasis of the numerical investigations is the optimization of the ventilation of the front and rear legroom.