To investigate the functional plasticity of the brain in childhood, the intelligence and language tests, brain CT and dichotic listening test were performed on 34 patients with hemiplegia. All 13 patients with left hemiplegia, 8 to 37 years old, showed a right ear dominance suggesting the lateralization of language in the left hemisphere. Among 21 patients with right hemiplegia, 7 to 22 years old, 16 patients who had suffered from the left hemispheric damage before 6 years and 1 month of age, showed the left ear dominance suggesting the lateralization of language in right hemisphere except 2 patients. On the contrary, 3 patients with right hemiplegia who had suffered from the left hemispheric damage after 6 years and 6 months of age showed a right ear dominance. It was concluded that the critical period for the dislodging of language lateralization in the brain from the damaged left hemisphere to the intact right hemisphere is the first half of 6 years after birth. Five patients out of 8 patients with right hemiplegia showed significantly lower scores in performance than in verbal WISC IQ. The compensation of language function rather than performance function was considered to be characteristic for the left-hemispheric damage in childhood.