We investigated red nucleus (RN) changes in patients with a cerebral infarct, using diffusion tensor imaging (DTI). Forty-nine consecutive stroke patients with pyramidal tract(PT) injury (mean age, 62.1 years; range, 41 to 75) and 46 age-matched normal healthy control subjects (mean age, 58.9 years; range, 41 to 75) with no history of neurologic disease were recruited. DTI was performed using a 1.5-T system during the early stage, after cerebral infarct (8-l21 days after onset). DTIs were acquired using a sensitivity-encoding head coil at 1.5 T. We measured fractional anisotropies (FAs) and apparent diffusion coefficients (ADCs) of RNs in the upper midbrain, and assessed the motor function of affected extremities. Mean FA of RNs in affected hemispheres was higher than in unaffected hemispheres (P < 0.05), however, no difference was observed between the unaffected control hemispheres (P> 0.05). However, mean ADC values of RNs were no different between control hemispheres and a patient's affected and unaffected hemispheres (P> 0.05). It appears that RN in affected hemispheres show elevated levels of neuronal activity during the early stage in patients with a corona radiata infarct, which suggests that RNs in affected hemispheres are used to compensate for PT injury.