OBJECTIVE To determine the route of the spinal cord at the thoracolumbar junction in a sagittal MRI taken in the supine position in tethered cord syndrome. METHODS We retrospectively studied the MRIs of 11 cases with tethered cord syndrome and the MRIs of 33 individuals without spina bifida were used as a control. When the anteroposterior diameter of the low intensity area behind the spinal cord on the sagittal T1 weighted images taken in the supine position was more than one third the anteroposterior diameter of the cord at T12/L1, the spinal cord was defined as being displaced ventrally. RESULTS Ventral displacement was found in 6 cases (VD group), but not in the rest (non-VD group). None of the control group showed ventral displacement. There was a tendency for motor weakness to be more severe in the VD group. CONCLUSIONS In some cases with tethered cord syndrome, the spinal cord takes a ventral route at the thoracolumbar junction even on MRI taken in the supine position. This finding may be useful in predicting the severity of the motor weakness in the lower extremity.