Movement of an affected arm after stroke is associated with a more bilateral pattern of brain activity than controls. We tested the functional significance of ipsilateral brain activation after stroke by temporary inactivation with transcranial magnetic stimulation (TMS). We applied TMS to the ipsilateral motor and premotor cortex (sites are indicated by yellow and blue circles). The behavioural effects of premotor cortex TMS are shown in the graph. In controls, TMS of ipsilateral premotor cortex only slowed simple movements when applied late (150ms). In patients, TMS of ipsilateral premotor cortex slowed movements early (100ms) after a cue to move. This demonstrates that the ipsilateral activation detected in patients is neccesary for recovered movement of the affected limb.