The neurologic outcome of 23 seven-year old children who had cord blood hyperviscosity was compared with that of children with normal cord blood viscosity in a randomised, controlled and blinded study. Viscosity was measured using a coaxial narrow-gap couette viscometer. Sixteen (69.6%) of the children with hyperviscous cord blood had a disability; this incidence being three times greater (22.7%) than in children whose cord blood was not hyperviscous (P < 0.01). In three children with cord-blood hyperviscosity, the disability was severe. No child had a severe disability with normal cord blood viscosity. Of the eight children whose cord blood was hyperviscous, but not polycythemic, six (75.0%) had a disability and in one child the disability was severe. These results demonstrate an association between cord blood hyperviscosity and later neurologic development. Cord studies are non-invasive and result in the rapid diagnosis of the neonatal hyperviscosity syndrome, so allowing earlier treatment. This may be crucial in altering the effects of hyperviscosity on the developing brain in the early neonatal period. Because the neurologic outcome of children was similar whether polycythemia was present or not, the prime factor was the viscosity and not the hematocrit level. We suggest it may be necessary to perform cord blood viscosity studies routinely.