A very small diameter wire is tethered from the apex of a delta wing and nominally aligned with the centerline of the leading-edge vortex. The wire can alter both the onset and structure of vortex breakdown. A technique of high-image-density particle image velocimetry allows acquisition of patterns of instantaneous and averaged vorticity and velocity, which reveal the relationship between: Advancement of vortex breakdown towards the apex of the wing; and corresponding changes of patterns of vorticity and velocity contours. The diameter of the wire is one percent of the core diameter of the pre-breakdown vortex. It is possible to alter the onset of vortex breakdown by as much as approximately one chord length of the wing. A critical parameter is the length of the wire, which is normalized by: The chord of the wing; or the distance to onset of vortex breakdown in absence of the wire. Once a critical length of the wire is attained, further increases in length have no effect on the onset of breakdown. This effect is interpreted in terms of abrupt changes in patterns of vorticity and streamwise gradients of velocity along the central region of the vortex. It is possible to attain a switch in sign of azimuthal vorticity and a wakelike region of the vortex, in absence of a stagnation point. (C) 2003 American Institute of Physics.