29 hydrocephalic infants (less than 30 months) who had shunts were studied with the 'Bayley Mental Scale' and with computed tomography. It was found that patients with a medical history indicating brain injury in addition to hydrocephalus had lower scores on the 'Bayley Mental Scale' than a comparable group whose hydrocephalus had been uncomplicated. Dilatation of the lateral ventricles was assessed by computing the ventricle-brain ratio (VBR) from the CT scan. The VBR was inversely correlated with the 'Bayley Mental Scale' score. Two thirds of the sample were reevaluated after 1 year; there was a significant decrease in VBR, but mental quotient was unchanged. These findings are in agreement with studies on older children in that the prognosis for cognitive functioning in hydrocephalic children is affected by brain injury. A correlation between brain mass and intelligence has been reported in older children with uncomplicated hydrocephalus, but not in groups with mixed etiology.