Hemispatial neglect is common after unilateral brain damage, particularly to perisylvian structures in the right-hemisphere (RH). In this disabling syndrome, behaviour and awareness are biased away from the contralesional side of space towards the ipsilesional side. Theoretical accounts of this in terms of hemispheric rivalry have speculated that the intact left hemisphere (LH) may become hyper-excitable after a RH lesion, due to release of inhibition from the damaged RH. We tested this directly using a novel twin-coil TMS approach to measure excitability within the intact LH of neglect patients. This involved applying a conditioning TMS pulse over left posterior parietal cortex (PPC), in order to test its effect on the amplitude of motor evoked potentials (MEPs) produced by a subsequent test pulse over left M1. Twelve RH stroke patients with neglect, an age-matched group of 8 RH stroke patients without neglect, and 10 healthy controls were examined. We found that excitability of left PPC-M1 circuits was higher in neglect patients than the other groups, and related to the degree of neglect on clinical cancellation tests. A follow-up found that 1 Hz repetitive TMS over left PPC normalised this over-excitability, and also ameliorated visual neglect on an experimental measure with chimeric objects. Our results provide direct evidence for pathological over-excitability of the LH in the neglect syndrome, as quantified by left PPC influences on left M1, with implications for possible treatment.