OBJECTIVE The purpose of this study was to evaluate hemispheric asymmetry of cerebral blood flow changes during various mental tests by applying transcranial Doppler sonography (TCD) to simultaneously monitor bilateral cerebral blood flow velocity changes. METHODS Twenty-one participants without cerebrovascular disease performed 3 left hemispheric tasks (reading, calculation, and color scaling) and 3 right hemispheric tasks (face recognition, space imagination, and line orientation). RESULTS Mean velocities of the rest and performing periods did not differ significantly between the left and right hemispheric tasks. Although greater acceleration of blood flow velocity was observed on the left than on the right in most of the 6 tasks except line orientation (mean left - right ratio difference [D(l-r)] ranged from -0.018 to 0.071), this difference was larger for left hemispheric tasks (mean D(l-r) ranged from 0.050 to 0.071) than right hemispheric tasks (mean D(l-r) ranged from -0.018 to 0.034; P < .001). Further comparisons of each pair of (ie, left and right) hemispheric tasks revealed that the most suitable left and right hemispheric tasks to show hemispheric asymmetry were reading and line orientation, respectively (P < .001). CONCLUSIONS Hemispheric asymmetry of cerebral blood flow changes during mental tests is demonstrable with TCD only when comparing the D(l-r) in response to suitable paired left and right hemispheric tasks.