One of the central questions in developmental biology is that of pattern formation in organs. Leaves of most plant species have distinct dorsoventral (adaxial-abaxial) polarity, which is formed early during leaf primordium growth at the shoot apex. Establishment of polarity is essential for the development of planar leaves. It has long been proposed that the shoot apex produces a chemical signal that promotes adaxial leaf fates and orients the dorsoventral axis. This proposed Sussex signal provides an explanation how initial dorsoventral polarity information is processed and transmitted. Here we show that a self-organized auxin flow from emerging leaf primordia axil to the shoot apical meristem establishes an auxin gradient along the boundary-adaxial-abaxial axis, which controls leaf dorsoventral polarity prior to and during leaf initiation. Local application of polar auxin transport inhibitors abolished this auxin flow and dorsoventral polarity of the related leaf, as suggested by mutant analysis. Furthermore, auxin application allowed correlation of leaf dorsoventral pattern formation with a gradient of auxin concentration in emerging leaf primordia. Such a self-organized auxin flow induces a low auxin zone early during leaf formation to establish the adaxial domain. Opposite to the original Sussex proposal, instead of a positive signal is derived from the meristem, we show a signaling molecule is departured from the primordium to promote leaf adaxial fates.