The functional development of the left and right cerebral hemispheres in 39 normal children, aged 18 days to 19 years, was studied by measurements of cerebral blood flow at rest, using single photon emission computed tomography (SPECT), at the Neuropediatric Department, Hospital Saint Vincent de Paul, Paris, France. Functional brain activity and blood flow were greater in the right hemisphere in children between 1 and 3 years of age, whereas the left hemisphere was functionally dominant at 4 years and later. The change from right to left asymmetry was due to a shift in activity of the posterior associative area. This shift of asymmetry appeared to be functionally related to the visuospatial abilities subserved by the right hemisphere in the first year followed by emergence of language abilities at 3 years of age, a left hemisphere function. 
COMMENT. Right hemisphere cerebral blood flow (CBF) is higher than left at 1 to 3 years of age, whereas left CBF is higher in older children. Right hemisphere of infants is functionally dominant whereas the left becomes dominant after 3 years, when language develops. These significant CBF asymmetries are detected in sensorimotor cortex, Broca’s area and the posterior associative regions which serve handedness and language.