The cognitive functioning, motor skills and behaviour of 5-year-old children with early-treated congenital hypothyroidism was assessed. The study group was 57 children with congenital hypothyroidism (CH) diagnosed by neonatal screening in N.E. and N.W. Thames regions between 1978 and 1981 along with 51 non-affected controls matched for age, sex, social class and language background. Small differences in I.Q. and behaviour between the patients and the controls were not statistically significant. However, children with CH showed significant deficits in motor skills (M 79.9 SE 3.7) compared to the controls (M 99.8 SE 4.0) (P = 0.0003). Deficits were particularly marked for balance. In addition, children with more severe hypothyroidism at diagnosis (Plasma thyroxine less than 20 nmol/l) did significantly less well in respect to I.Q. and motor skills than those with less severe hypothyroidism (plasma thyroxine greater than 60 nmol/l). These findings provide further evidence for the importance of the severity of hypothyroidism in determining the outcome for intelligence and motor skills in children with early-treated congenital hypothyroidism. Deficits in motor skills, particularly in relation to balance, suggest that early impairment of the vestibular system may occur despite early treatment.