Results of numerical flow field and aerosol deposition calculations in bronchial bifurcations are very sensitive to variations in physical and geometric boundary conditions. In general theoretical models are based on relatively crude geometric approximations due to mathematical and numerical difficulties. Here we define a physiologically realistic bifurcation (PRB) geometry for experimental and theoretical three-dimensional fluid dynamics and particle deposition studies in branching airways. The most significant differences to previous idealized bifurcation models are the reduced skewness of the inspiratory flow, smaller secondary velocity components, and regions of reverse flow in the vicinity of the carina during inspiratory as well as expiratory phases.