The degree of Wallerian degeneration (WD) in the corticospinal tracts seen with magnetic resonance imaging (MRI) was correlated with the distribution and severity of congenital hemiplegia in 20 children aged nine months to nine years. All the children had hemispheric lesions diagnosed with ultrasound in the neonatal period: MRI and clinical assessment were performed from nine months to nine years of age. Hemiplegia was graded as mild, moderate or severe and into predominantly upper or lower limb distribution. WD was assessed by the presence or absence of signal intensity changes in the internal capsule on inversion recovery and spin echo sequences and by the asymmetry of the upper brainstem. The degree of asymmetry was estimated by measuring the cross sectional area (CSA) of the brainstem at three levels and calculating the ratio of the measurements between the side of the lesion and the unaffected side. Infarct size was estimated from the CSA of the infarct at the maximum site of the lesion. Both measurements were correlated with the severity of outcome and the site of involvement. There was a better correlation between severity of outcome and brainstem asymmetry (p = 0.003) than severity of outcome and infarct size (p = 0.02). There was also a significant correlation between upper limb involvement and brainstem asymmetry (p = 0.01). As WD estimated by brainstem asymmetry appears early and is easy to measure, it may be a good marker to estimate later impairment in infants with predominantly unilateral hemispheric haemorrhagic/ischaemic lesions diagnosed in the neonatal period.