From the point of view of physical modelling, phonation is the result of a complex interaction between the airflow through the glottis and the mechanical reaction of the vocal folds. During "normal" phonation, one can reasonably assume that both vocal folds are oscillating in phase and thus that glottis forms a symmetrical channel for the airflow. A number of voice pathologies, such as unilateral laryngeal paralyses, or involving tumours or lesions, are however clearly causing an asymmetrical motion of the vocal folds. All existing studies about such pathologies are focussing on the asymmetrical mechanical aspect of the problem whereas the possible effects of this asymmetry on the flow is completely overlooked. In an attempt to evaluate the relevance of such an assumption, we present in this paper a systematic study of the symmetry of the flow through symmetrical and asymmetrical replicas of the vocal folds.