The authors report a girl with left-sided hemiparesis and unilateral hydranencephaly of the opposite side. Her psychomotor development has been far better than expected--she is only mildly delayed. Hemihydranencephaly is a rare anomaly: only five other cases are reported in the literature, three of whom were also mildly delayed. The fourth patient had multiple congenital defects combined with the hemihydranencephaly and was severely retarded, and no data on the fifth patient were provided. It may be that the prognosis for hemihydranencephaly is better than for most other destructive encephalopathies, which generally carry a poor prognosis for psychomotor development.