Abstract Spatial cognition is right-hemisphere dominant in right-handers, but hemispheric laterality in left-handers is not fully understood. Using near-infrared spectroscopy, we compared cerebral activations in the frontal and parietal lobes during a mental rotation task between seven healthy right-handed and seven healthy left-handed women. Cerebral laterality during the spatial cognition task was evaluated as balance in the extent of activation areas between the two cerebral hemispheres, using the right-hemispheric dominance index (RI). RIs of right-handers showed right-hemispheric dominance (RI > 0) in both frontal (RI = 0.31 ± 0.25) and parietal (RI = 0.28 ± 0.37) lobes, while left-handers showed slight left-hemispheric dominance (RI < 0) in both frontal (RI = −0.13 ± 0.18) and parietal (RI = −0.22 ± 0.22) lobes. The left-handers exhibited significantly larger amplitudes of activation at the channels overlying the left-superior parietal lobule, whereas the right-handers did not show such amplitude differences. These findings suggest a difference in cerebral hemispheric laterality for spatial cognition between left- and right-handers.