Motor functional recovery after stroke may be attributable to cerebral reorganization. We used near-infrared spectroscopy, which measures non-invasively the changes in oxy- and deoxy-hemoglobin concentrations in response to neural activation, for monitoring cerebral activation in stroke patients, and investigated the longitudinal changes in functional laterality of activations in the primary sensorimotor cortex during unilateral audio-paced (1 Hz) hand movement. We examined five ischemic stroke patients (4 females and 1 male, 52-67 years old) with mild to moderate hemiparesis at acute stages and chronic stages at least 1 month later. Normal subjects (3 females and 2 males, 47-63 years old) were also included. Unilateral hand movement activated predominantly the contralateral primary sensorimotor cortex in the normal subjects and the stroke patients when they moved unaffected hand. Affected hand movements activated bilateral sensorimotor cortices early after stroke (< 25 days of stroke onset), whereas the activation pattern returned toward normal at later periods (> 35 days). The contralaterality index (0.34 +/- 0.12 in normal control) was reduced at early periods (0.00 +/- 0.03, p < 0.01) after stroke, and returned to normal (0.35 +/- 0.24) as motor function recovered. These findings suggest that a transient increase in motor activation in the ipsilateral intact hemisphere within 1 month may play an important role in the recovery from motor dysfunction after stroke.