Positron emission tomography (PET) was used to identify the brain regions activated during the 'inner signing' of sentences in subjects who were profoundly deaf and normally communicated using sign language. Although this appeared to involve the internal representation of hand and arm movements in space, it activated the left inferior frontal cortex rather than visuo-spatial areas. The activated region corresponds to that engaged during the silent articulation of sentences in hearing subjects. This suggests that 'inner signing' is mediated by similar regions to inner speech, and is consistent with neuropsychological data implicating the left hemisphere in the generation of sign language.